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Fantastic Underworld (trope)

In real life, even the largest caves are by necessity fairly localized things, as they require specific conditions to form — you will usually only find them in soft, easily eroded rocks and following the movement of rivers or at least aquifers, which means that most exist close to the surface and within bounded geographic areas. Not so in fiction, where it often appears that the Earth's crust and depths alike are often filled with vast networks of spacious caverns.

The landscape typically starts out as a magnified version of a regular cave, with miles of winding tunnels, caves, and abysses of craggy stone. More unusual sights usually lie further in and deeper down. Forests of giant crystals, great underground rivers, lakes, and seas, rivers and waterfalls of lava, and ancient ruined cities are all common features down here. Growths of crystals, gems, and immense geodes may produce entire Crystal Landscapes. Fungi, often of incredible size, replace plants and may form extensive fungal jungles, although tropical or alien vegetation may also appear in areas with some form of lighting. Darkness and gloom are usually the order of the day, but light may be provided by lava, Glowing Gems in the cavern walls, electromagnetic phenomena, or the like.

Lost Worlds are also common sights, usually in the form of valley-like caverns or underground seas lit by some sort of false sun at their roofs. They are often filled with jungles of lush vegetation, and home to dinosaurs, sea monsters, and primitive cavemen.

As a rule, deeper caverns will contain stranger sights and natives than shallower ones. It is not uncommon for an explicit "layering" to occur, with distinct strata of underground lands that grow increasingly strange and perilous as one heads deeper down. If this is the case, the very bottom-most layer will usually be a roiling, seething sea of lava, a home of unimaginable terrors, or both.

Mole Men, The Morlocks, and fungus people are common inhabitants of these regions. In fantasy, expect instead to find dwarves, goblins, dark elves and Bat People making their homes down here. Entire Underground Cities can be found down here, filled with life or long in ruins, unless the locals live as primitive tribes in the stony wilderness. In most settings, Big Creepy-Crawlies, huge, monstrous bats, and strange, pale relatives of surface animals are common parts of the local fauna, as are Living Dinosaurs in a lost world.

Narratively, this is almost always an exotic and perilous landscape. Stories rarely start down here, although getting to or navigating through this underworld may be the meat of an adventure story — indeed, this is a staple of the subterranean fiction genre. This is also a part of why the people and creatures living down here are usually so strange and unusual — whatever may make its home miles beneath the light of the sun certainly won't be anything like what you'd encounter on the surface world or a shallow tunnel.

If this subterranean landscape is so vast and habitable that it effectively has a sky, it qualifies as a Hollow World. Compare Mega Dungeon, as a sufficiently large or deep example of that trope can overlap with this one. Compare Underground Alien Civilization, for when an alien culture lives in subterranean areas of another planet. Definitely not to be confused with The Underworld, which has more to do with the afterlife than with anything underground.

Examples:

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Anime & Manga 

  • Made in Abyss: The Abyss. It's for the most part well illuminated because of the fluorescent mists that pervade it. The farther down someone travels, the more elaborate its contents become, with upside-down forests, enormous plants, flying serpents, and buried cities existing in the depths. People risk death to retrieve the incredibly valuable artifacts found on the lower Layers, although, past a certain point, the Maybe Magic, Maybe Mundane Curse of the Abyss means that return becomes fatal.

Card Games 

  • Magi-Nation: The Underneath is a large cave system home to a large city, fungus jungles, and fungal monsters.

Comic Books 

  • Atomic Robo: There's an extensive, undetectable planet-wide cave network populated by an alien silicon-based ecosystem, created when debris from Theia were captured by Earth's gravity and sank into the crust billions of years ago.
  • Bone: Located in the mountains is Tanen Gard, the sacred burial ground of the dragons. The dragons have gone underground during the war, and trespassers are forbidden from entering, else they end up being penalized with death. While the heroes manage to cross it successfully, Thorn and Fone Bone willingly enter Tanen Gard to find the Crown of Horns in order to put an end to the war. Tanen Gard is a cavernous maze that goes underground, complete with stairs, walkways, and even rushing water that can fill a pit. When Fone and Thorn require the Great Red Dragon's help to escape, the dragons they are chased by resurface by breaking through the ground.
  • Marvel Universe: There are a number of cavernous underworlds deep beneath Earth. Subterrannea is inhabited by yellow-skinned, weak-willed humanoids and ruled by various humans who've gone there over the centuries — most notably the Mole Man, a Fantastic Four villain. There are also the Lava Men from The Avengers and lots of giant monsters. Supposedly, most of these beings were genetically engineered ages ago by the Deviants, another subterranean race who were themselves genetically engineered by Celestials.

Fairy Tales 

  • "Reygoch": A vast network of caverns, passages and subterranean streams lies beneath Frosten and the endless snowy plain surrounding the ancient city. While exploring the caves, Curlylocks and Reygoch find treasures which had been swallowed by the earth long ago: castles, ships, weapons, buried treasures...
  • This is a feature of tale type ATU 301, "The Three Kidnapped Princesses" (in the East Slavic Index, "The Three Underground Kingdoms/Tsardoms"): three brothers follow a creature or monster to a well, but only the youngest is brave enough to go down the well. Down there, the youngest brother finds three princesses (who may have been kidnapped in a prologue to the action in some variants, or are simply found there in others), each inside a castle made of metal (the first of copper, the second of silver, and the third of gold). Each of the princesses is being held captive by a monster (three in total).
    • Three Men of Power: Evening, Midnight and Sunrise (Russia, retelling) - three princesses go for a walk in the garden and a whirlwind snatches them; three brothers (Evening, Midnight and Sunrise) go to look for them; Sunrise descends the pit and finds the princesses in three castles (of copper, silver and gold), each guarded by a snake.
    • The Norka (Russian tale collected by Alexandr Afanasyev) - the youngest prince, Ivan, hurts the creature named Norka and follows it down to its lair beneath the earth; down there, he meets a fully-equipped, talking horse, and rides to the three palaces of the Norka's human sisters, who are enchantresses. Each of their castles, the first of copper, the second in silver, and the third golden, are fully furnished with a dining room and bedroom. The Norka's sister who lives in the golden palace says her brother is asleep in the blue sea. Later, after Ivan's brothers betray him and strand him in the lower world, he protects a nest of chicks from a storm raging on in the underground.

Fan Works 

  • Antipodes: Vast networks of caverns are implied to run beneath most of the planet's surface. Some of the survivors of Equestria's collapse fled here to avoid to increasingly dangerous conditions on the surface, building vast bunkers and drawing on underground bodies of water, but most of the cavern systems are unexplored wilderness full of monsters.
  • The Land of Dragons and Dungeons: The Deep Caverns are a network of caves and abysses that runs beneath every continent and ocean, reaching down to the molten core of the world and occasionally breaching to the surface through connections to ordinary cave systems. The dwarves inhabit the upper border, typically on the surface cave side; the drow live much deeper down, in cities among the eternal darkness. The bulk of the cavern network is a hostile wilderness haunted by terrible monsters, from simple cloakers and wandering slimes to ancient, nameless horrors that not even the drow empires wish to rouse.
  • Nine Days Down: There is an extensive network of caverns and tunnels beneath the surface of Tartarus, crossed by underground rivers and home to monsters such as predatory shrimp, clans of wights, and unique and dangerous entities sealed in isolated caverns. When he first comes to the surface, Bait the wight is profoundly unnerved by the experience of being in a seemingly endless open space without any walls or ceiling.
  • The Palaververse: An extensive underworld exists beneath Theia's surface, permeating the planet's entire volume down to its core in a maze of tunnels and caverns, underground rivers and plumes of lava rising from seas of magma. The uppermost layers are (relatively) tame and home to the underholds and mines of the Diamond Dogs, but the deeper levels are home to things like sapient colonies of fungi, living patches of darkness or poisonous gas, blind and flightless cave dragons, and the Dwellers Below, Eldritch Abominations spawned by primal and chaotic magic. Supposedly, the Diamond Dogs originally lived in the planet's very heart, but migrated upwards to their current location after an apocalyptic "Creation War" between unspecified empires turned the planet’s core into a sea of molten rock.

Films — Animation 

  • Atlantis: The Lost Empire: To reach the lost city of Atlantis, the expedition passes through an underwater cave to emerge in a network of caves somewhere beneath the Atlantic Ocean. After a difficult journey through caverns filled with ancient ruins, deep ravines, and monstrous arthropods, they eventually reach the cavern where the ancient city itself persists alongside ancient animals long extinct on the surface.

Films — Live-Action 

  • The Lord of the Rings: Moria, the subterranean dwelling of the Longbeard Dwarf clan under the Misty Mountains, which is now a lightless ruin home to hordes of goblins, trolls, and an ancient demon.
  • Maleficent: Mistress of Evil: When it became clear they could no longer fight the humans, all of the Dark Fey (except Maleficent) retreated to their "nest of origin", a huge cave in the middle of the sea that, possibly due to magic, has mini-ecosystems from around the world.
  • MonsterVerse: A major setting of the series is the Hollow Earth, a vast and thriving subterranean world beneath the Earth's crust, accessible by vortexes, where kaiju originate from. The Hollow Earth also has its own even deeper sub-level, known as the Subterranean Realm, home to even larger and more monstrous creatures.
  • Star Wars: In The Phantom Menace, the planet Naboo is riddled by a tangle of flooded caverns inhabited by gigantic fish and monstrous leviathans, which is used by the Gungans, who live in its uppermost reaches, as a swift but risky path through the planet. The humans living on the paradisiacal surface are apparently completely unaware of this, as they happily build their cities and homes along the shorelines of bodies of water that lead directly into the monster-filled abyss.

Literature 

  • Age of Fire: The Lower World is a network of tunnels and cave systems said to be home to most of the surviving dragons, alongside the reclusive and physically monstrous "demans". It's often mentioned during the first two books but only rarely and briefly explored. In contrast, most of the third takes place there; it turns out that the Lower World is not only absolutely massive, but may well be just as populous as the Upper.
  • All Tomorrows: The people of one world tried to hide from the Qu in continent-sized bunker complexes. The Qu found them anyway, and filled the bunkers with a complex troglodytic ecology, fueled by trickles of nutrients and water from outside, including vast forests of mushrooms, giant pale insects, huge bats, crocodile-like fish patrolling underground waters, and the humans' blind, almost batlike descendants.
  • Artemis Fowl: The Earth is permeated by an extensive system of caverns and vertical abysses leading to the planetary core, which the books claim are necessary for allowing magma flares to expend their energy in without tearing the crust and mantle apart. The fairy folk retreated there after being forced away from the surface, and now mostly live in Haven City within a massive cavern. Most of the rest of the tunnels are empty except for small outposts and wandering trolls.
  • The Chronicles of Narnia: A series of three consecutively deeper realms is encountered in The Silver Chair:
    • The Deep Realm is a large cavern found by passing through long dark tunnels that open far to the north of Narnia. It contains forests of giant fungi and great piles of slumbering birds, bats, and dragons, alongside the giant recumbent form of Father Time, all of which are sleeping until the end of the world. In The Last Battle, when the world does indeed end, these creatures awake, come streaming out of the Deep Realm, scourge the world of plant life, and die.
    • Underland lies beneath the Deep Realm. It is a single giant cave the size of Narnia itself, and is mostly filled by a huge body of water called the Sightless Sea. Here is also the dark fortress ruled by the Lady of the Green Kirtle.
    • About six thousand feet further down is even deeper realm called Bism, where gold and gemstones are alive and where dragons and salamanders live among rivers of lava. This is where the Lady's enchanted Earthmen slaves were taken from. Once roused from their enchantment, they find Underland, which they call the Shallow Lands, to be uncomfortably close to the sky...
  • The Descent: There is an extensive labyrinth of tunnels and passages stretching throughout the sub-surface of the entire world, inhabited by several species of troglodytic hominids who have adapted to their underground conditions.
  • Dinotopia: The World Beneath is a vast system of caverns running beneath most of the island. The dinosaurs hid there to escape the end-Cretaceous extinction. Later, refugees from the sunken city of Poseidos hid there to try and restart their factories that built their robotic beast-vehicles, but they failed, abandoning the "strutter-works" and handing over their treasure to the tyrannosaurs for safe passage through the Rainy Basin. Besides vast abandoned complexes and flooded ruins, the World Beneath is also filled with natural wonders, including complex speleothems and caves filled with crystals.
  • The Dream-Quest of Unknown Kadath: There is a vast underworld beneath the Dreamlands that Carter passes through twice. There are a number of distinct regions within it, such the subterranean plateau around the Peaks of Throk, a range of granite mountains, where the Ghouls and Night-Gaunts live; the deep Vale of Pnath beneath it, where the Ghouls throw the remnants of their meals and huge, wormlike Bholes burrow and writhe; the cavern where the Gugs' city is; and the Vaults of Yin, where the ghasts dwell. The Peaks and the Gugs' city are lit by a faint grey phosphorescence, but the Vale and Vaults are dark as pitch. The whole thing seems to lie at least somewhat apart from the Dreamlands proper, as both the dreaming world and the waking world can be accessed through its tunnels.
  • Fragment: The sequel, Pandemonium, introduces the titular Lost World, a colossal and partly submerged cave system beneath the Ural Mountains that has been isolated for over three hundred million years. A complex ecosystem based on bioluminescent fungi has formed and is inhabited by huge and voracious invertebrates, including amphipods the size of hippos and zeppelin-like flying cephalopods as massive as whales.
  • Goblins in the Castle: Goblin Land, or Nilbog as it's properly known, is a city hidden deep beneath the surface in a cavernous area, lit by glowing fungus.
  • The Iron Teeth: A large monster-filled network of caves called the Deep is mentioned, but only briefly explored. Small insects called Harvesters live there and build vast mushroom farms.
  • Journey to the Center of the Earth: The events take place primarily in a complex network of caves beneath the surface of the Earth, accessible through volcanic vents. After passing through a long series of lightless, empty caves, the protagonists find a subterranean ocean, which they call the Lidenbrock Sea. It fills a single titanic cavern, is lit by constant electrical storms, and is inhabited by prehistoric sea monsters.
  • Kane Series: In "Two Suns Setting" there is the strange cavern system hidden beneath dwarf king Brotemllain's burial cave. It's inhabited by huge, white, blind beasts, including a yard-long white cockroaches, as well as sabretooth tigers.
  • The Lord of the Rings: The ancient Dwarf city of Khazad-dûm was delved deep beneath the Misty Mountains, and since falling into ruin has become a dark underworld home to goblins, trolls and an ancient demon. There are even deeper abysses further down, below even the deepest mines of the dwarves, where nameless things gnaw upon the living rock.
  • Memory, Sorrow, and Thorn: The labyrinthine tunnels beneath the Hayholt, including the ruins of Asu'a, are a major story element. Simon is forced to traverse them twice during his journey, both times representing his "descent into darkness"". Other characters visit the tunnels as well, including the Sithi (and Norns) near the climax.
  • Mother of Learning: The planet is laced with a series of magical caverns extending, in legend, to the very heart of the world. They're more dangerous the deeper you go.
  • "The Mound" by Zealia Bishop and H. P. Lovecraft: A Spanish conquistador descends into an underground world of caverns bathed in blue light called K'n-yan, home to a cruel and sorcerous civilization. In true Lovecraftian fashion, this world proves only the first of a succession of otherworldly abysses. Below K'n-yan is the grotesque, red-lit world of Yoth, filled only with the ruins of an inhuman people, and below that is the nightmare realm of the lightless N'Kai.
  • Otherverse: The Warrens are a Realm that consists of underground tunnels that are the home and birthplace of goblins and goblin-adjacent Others such as Heaplings and Fleshmonglers. It is further described as the place where the foulest bits of humanity end up, and is fittingly extremely filthy. If you go deep enough, it's even possible to end up in the Abyss, a different Realm entirely.
  • Star Trek: In the Rihannsu series, the Romulan/Rihannsu colony world Ysail is famed for its vast Saijja Caverns, a network of caves so vast and deep that they cannot be accurately scanned from orbit and have never been mapped. When the Rihannsu government becomes more oppressive in the later books, most of the Ysailsu retreat to the deepest explored parts of the caves, which become an excellent base for La Résistance.
  • The Underland Chronicles: Underland is a vast cave system beneath the eastern coast of North America — its one known entrance is in New York City — whose caverns and winding tunnels are home to entire nations of gigantic rats, bats, cockroaches, spiders and similar animals, alongside white-haired humans with translucent skin who moved down there to escape the Sun's eventual cooling. The Underland is home to many varied environments beyond to expected barren caverns, including volcanic wastes, an underground jungle infested with carnivorous plants, and an underground ocean haunted by sea monsters.

Live-Action TV 

  • The Young Ones: "Boring": Parodied. An underground world where interesting things happen all the time (and as a result the rulers are bored of interesting things happen and want to meet a really boring person) exists under the protagonists' house. Neil is about to enter their realm when the grave he's digging for himself almost reaches down to their level, but he quits just before he gets there.

Music 

Puppet Shows 

  • Fraggle Rock: The Fraggles live in an elaborate world of caverns, with the entrance in Doc's workshop. Or beneath the lighthouse, in the UK version. In both cases, exactly where the Gorgs' garden is in relation to the human world is completely unexplored, although the events of "Uncle Matt's Discovery" and "Red's Blue Dragon" strongly imply, through some otherwise improbable tunnels to places like Australia and a world with a dragon guarding golden apples, that holes to Fraggle Rock lead through and to alternate dimensions with no surface-related logic, and the Gorgs probably live in a different dimension from us. It's possible that Fraggle Rock isn't even physically located under Doc's workshop or the Captain's lighthouse, but that the hole is simply another portal.

Roleplay 

  • A Sword Without A Hilt: The Underdark is a realm deep below the surface of the earth, with entire societies of intelligent races that live out their lives without ever seeing the sky. Most prominent among them are the Drow, otherwise known as the Dark Elves, beholden to Lolth the Spider Queen.

Tabletop Games 

  • Claim the Sky: Inner Earth is a network of tunnels and caves that includes Azari ruins, a Lost World cavern with Living Dinosaurs, and Mole Men (called "undermen"). One of the caverns lies underneath the Sahara Desert, and multiple entrances are buried in the sand.
  • Dungeons & Dragons: A subterranean world is a common fixture throughout the various settings and editions.
    • Eberron: The underworld is called Khyber after the primordial dragon-god from whose body it was purportedly created. In some early material, Khyber was a world of pitch blackness, ruled by Eldritch Abominations from Xoriat, the realm of madness, where cults of surface-dwellers worshipped the mutagenic energies that emanate from its black crystals. Later changes have reworked it so that while there is a "mundane" underworld, where you find subterranean monsters and kobold settlements, that's mostly the least terrible part of the area, unless you're in the Mror Holds (where those subterranean areas are actively occupied by swarms of aberrations); the real trouble in Khyber comes when you start breaking through into the demiplanes, which allow you to find monstrous forests full of soul-absorbing fruit, citadels of flesh or eyes ruled by Humanoid Abominations, the heart-realms of imprisoned Overlords, vaults full of goblins maintaining the culture of an empire that fell thousands of years ago and other fascinating and/or awful experiences for your adventuring party. Notably, these don't touch the world in predictable ways; Belashyrra's prison-realm can be accessed from both the Shadow Marches and Xen'drik, not because it's that big but because both areas host portals to it.
    • In Forgotten Realms and Greyhawk, it's called the Underdark, and both use it in more or less the same way — a very hostile environment filled with ancient evil races and bizarre monsters, and the traditional home of the drow.
      • It's common for entire cities to exist in the caverns — Forgotten Realms, for instance, has Skullport directly under the city of Waterdeep, where it serves as a trade hub between the surface world and the Underdark proper, and the Garden City of Fluvenilstra, built into the giant mushrooms and other magically enhanced plants that dominate the local landscape and home to a robust population of Mushroom Men.
      • There's also the Feydark, the Feywild's version of the Underdark; like all things in the Feywild, it takes what exists in the material world and exagerates it; the Feydark is even more cavernous than the Underdark, and filled with lush vegetation; it's mentioned that to denizens of the Underdark, the Feydark seems like a paradise. Unfortunately, it's also home to the Fomorians, a race of insane cyclopes that are extremely hostile to other forms of life.
      • In 4th edition, legend has it that it most large tunnels in it were carved out by the blind, agonized strugglings of Torog, God of Imprisonment and Torture, to escape after he was entombed alive during the Dawn War against the Primordials, smashing through the dimensional walls between the mortal world, the Feywild and the Shadowfell in his efforts.
    • If being a Hollow World isn't enough, Mystara is also home to the True Broken Lands, a network of interconnecting cavern systems underlying the barren surface-world Broken Lands. Like its aboveground namesake, the True Broken Lands are divided among multiple species of hostile humanoids, from imperialistic kobolds to crude, ravenous trolls. Much deeper down, the Hollow World territory of the Shattenalfen is similarly riddled with interlinked cave networks.
  • Exalted:
    • Extensive systems of caverns, tunnels and abysses extend beneath much of the Flat World of Creation, and have been used for most of its history as a dumping ground for creatures that its various rulers didn't want to deal with — among other things, failed Primordial experiments, the Primordials' own surviving servants following the Exalted's rebellion, and various monsters during the ages of Exalted rule all wound up down there, and are known collectively as Darkbrood. The Mountain Folk also live in city-states within the caverns, locked in a Forever War against the Darkbroods. This was caused by the Solar Exalted, who grew jealous of them, and forced Autochthon (their maker) to geas them into staying underground unless an Exalted said they could come up. The cavern systems take on elemental aspects when approaching the Elemental Poles much like the surface world does — the Western caves beneath the Great Ocean, for instance, contain a large underground sea, while as one approaches the Pole of Wood the caves become filled with forests of giant mushrooms.
    • The Labyrinth is a particularly nightmarish version of stretching beneath the Underworld. Its appearance can vary significantly from place to place and traveler to traveler, but it consistently manifests as a mazelike warren of tunnels and caverns dotted with bizarre cavern-cities, haunted by spectres and hekatonkhires, and growing steadily more hostile and outlandish the deeper one heads until you eventually find the Neverborn's tomb-bodies.
  • In Nomine: The Catacombs, the domain of David, the Archangel of Stone, are a version of this running beneath Heaven itself. They begin as worked chambers and tunnels, before giving way to natural caves as one heads deeper, and lack the otherwise all-pervasive light that fills Heaven; they remain Heavenly, however, and the darkness promotes calm and contemplation. The deepest reaches are home to beings that rarely if ever come up to Heaven's surface; these include angels, shaped with long limbs, large eyes and pale colors to fit their home, alongside strange insects, blind fish, and bizarre grotesqueries — most of these creatures are the souls of subterranean animals, while a few are prototype animals from the early days of creation or lesser spirits who tend to the caves; some are also angels punished for some transgression, or unique creatures made by David for a specific purpose.
  • Lamentations of the Flame Princess: The expansion Veins of the Earth features an endless labyrinth, the titular Veins, miles below the earth, so deep that light has never touched it and full of things that defy explanation. Treasure is plenty down there, but food and light sources are worth their weight in gold.
  • Mechanical Dream: There are immense systems of caverns and tunnels running beneath the world, many holding full-sized rivers, lakes and oceans.
  • Middle-Earth Role Playing: Moria is given an extensive description, showing it as a layered, complex labyrinth of mines and underground ruins home to tribes of orcs and trolls, flocks of flesh-eating bats, restless undead, and serpentine dragons that shun the light of day, although its depths also hide the treasures of the ancient Dwarves and natural wonders in the deeper caves. The deepest levels of Moria are connected to the Under-Deeps, a lightless realm that has never been explored but which is believed to extend beneath the span of the Misty Mountains and where terrible monsters lurk.
  • Pathfinder: The world beneath is known as the Darklands and draws inspiration from pulp fiction of the early 1900s, such as Edgar Rice Burroughs's Pellucidar. It's divided into three layers:
    • The uppermost layer, Nar-Voth, consists of extremely deep and extensive but otherwise fairly normal caverns (which are often isolated and do not form a continuous network), and is the most familiar layer to the surface-dwellers. While most of it is sparsely populated stony wilderness, it is also home to fairly normal humanoids like goblins, xulgaths and hryngar, and some less normal ones like deros and mongrelmen.
    • Note that dwarves in Golarion invert the usual "fled underground or were banished there for their crimes" facet of this trope, as they started out subterranean and, in fact, undertook a massive divinely-ordained crusade (the Quest for Sky) to find and fight their way to the surface world. Duergar are the ones who stayed behind. Some other races native to the Darklands, like troglodytes and especially orcs, were unknown on the surface until the dwarves' militant migration chased them there from beneath.
    • Sekamina, the middle layer, consists of much larger and more thoroughly interconnected caverns and tunnels, extending beneath most of the world’s surface. It is home to more reclusive and dangerous races like the serpentfolk and ghouls, who rule true underground empires, alongside monsters such as morlocks and gugs. It has more exotic terrain than Nar-Voth, including fungal forests, volcanic caves and a true subterranean sea.
    • Finally, the deepest level — the Vaults of Orv — is as much a subject of fear and mystery for the people of Nar-Voth and Sekamina as the Darklands as a whole are for surface-dwellers. It consists of massive caverns the size of nations, some dark and others lit by artificial suns or glowing crystals, containing environments such as deserts, rainforests, ruined cities, mountain ranges and a vast subterranean ocean. The Vaults are inhabited by a great variety of monsters, ranging from Living Dinosaurs, dragons, monstrous arthropods and the undead to a variety of unspeakable horrors. They did not form naturally, but were excavated by a powerful earth elemental Precursor race called the Xiomorn to host their evolutionary experiments.
    • The Darklands also extend beneath Tian Xia in the far east, although there they are home to creatures such as oni, the denizens of Leng, haunted clockworks, Rat Men and cave giants rather than the races of the western Darklands.
  • Space 1889:
    • Luna's surface is barren and lifeless, but a large canyon on its far side gives access to an inhabited inner world. After passing through a series of flooded caverns, an explorer reaches a continent-sized cavern mostly filled with water except for a number of large dry shelves and islands projecting from its sides. These are home to large fungal forests and to the city-states of the Moon Men, who descend from survivors of Vulcan, the long-destroyed fifth planet, that sought refuge inside Luna's depths and have now long forgotten their origins or the nature of the external world.
    • The World Beneath the World is a complex system of caverns and tunnels running beneath the global jungles of Venus. It's formed from the constant growth of planet's vegetation burying older layers of the jungles and swamps beneath newer growth and rotting humus, forming a spongelike system of subterranean cavities supported by the petrified remains of ancient trees. This underworld is home to a complex ecology supported by the constant percolation of organic materials form above, mostly consisting of oversized arthropods, tangled fungal growths, and sundry bioluminescent organisms. It's also home to the Gri, eyeless, albino relatives of the Venusian Lizard Folk who have been eking out an existence in the caves for generations and have only sparse contact with their surface relatives and the human colonists. The World Beneath the World does not form a single world-spanning network; most of it consists of solitary caverns and small cave systems, and although complexes as large as Australia exist these are not extremely common. However, some speculate that even deeper caves exist and connect the shallower cave systems to form a titanic underworld, chiefly due to the uniform nature of the various cave systems' fauna.
  • Warhammer Fantasy Battle:
    • The Dwarfs and Skaven both live primarily underground, and over their histories have created extensive systems of tunnels and underground cities for travel and habitation. The Dwarfs created immense fortress-cities known as Karaks and a subterranean highway system called the Underway, while the Skaven created a rambling, chaotic network of tunnels called the Under-Empire. Both exist primarily beneath the Old World's mountain ranges, although the Under-Empire's tendrils stretch beneath most of the world. The two systems have multiple connections dug in secret by the Skaven, in addition to intersecting various natural cave systems; millennia of wars and cataclysms have also ruined most of the Dwarf cities and tunnels. The result is a continents-spanning maze of tunnels, caverns and lightless ruins infested with tribes of orcs and goblins, trolls, fungus monsters, dragons and worse, all beneath the feet of entirely unsuspecting surface-dwellers.
    • Much deeper abysses are mentioned to exist deep beneath the diggings of Dwarfs and Skaven, hiding enormous blind horrors, vast Blob Monsters and cities of ghoulish things where no light has ever shined. One of these areas is Underworld Sea, a vast labyrinth of flooded caves and tunnels stretching beneath the Dark Elven realm of Naggaroth. It's poorly explored, difficult to navigate, prone to floods and cave-ins and populated by ferocious monsters, and there are rumors that the ruins of a lost civilization exist within its depths.

Toys 

  • BIONICLE: Onu-Wahi was the region of Earth on the island of Mata Nui; unlike the other five Wahi on the surface, it didn't have definable borders, instead being comprised of a vast network of caverns and mines stretching under the entire island. Onu-Matoran were always expanding it, too, so it became a high-way system for the islanders to travel more quickly than they could above ground.

Video Games 

  • Age of Wonders: Maps can have two underground layers, the Caverns and the Depths, which can be accessed using caves. In Age of Wonders 4, provinces in these layers are separated by walls of dirt and rock — rock walls are impassable, but dirt walls can be excavated to reveal buried provinces and open new paths, at risk of spawning stacks of marauders.
  • Astroneer: All planets and moons have extensive, multi-layer cave networks, full of dangerous plants and valuable resources.
  • Caves of Qud: The titular Caves are host to astounding treasures and terrors. Non-story dungeons allow you to descend until you hit lava floes, and you can cross the game world entirely underground if you wanted to.
  • Dark Souls II: The Grave of Saints, Gutter, and Black Gulch are series of catacombs, sewers, and caves far beneath the hub town of Majula filled with rat kingdoms, mutants, and other strange creatures.
  • Dragon Age:
    • The Deep Roads are a vast subterranean network of artificial caverns and passages dug out by the dwarves in the heyday of their civilization. It had once consisted of hundreds of underground cities named "thaigs" and the miles-long tunnels connecting them, but, since the First Blight, all but two of them have been overrun by the darkspawn. One thaig, Orzammar, is the capital of modern dwarf civilization and is visited in Dragon Age: Origins.
    • Dragon Age: Inquisition: The Descent DLC leads the Inquisitor and their party far, far below the Deep Roads, which are generally considered the deepest explored reaches of the setting, into a fantastic underground world of gigantic caverns full of bioluminescent life and even a lost ocean. It's also home to an entire lost clan of dwarves wielding insanely advanced technology, and to the Titans, absolutely immense lifeforms whose mere movements trigger earthquakes on the surface, and whose blood is lyrium, the substance all surface magic is dependent on. The Inquisitor only explores the uppermost reaches of a Titan, implying that the sub-Deep Roads cave systems descend even deeper into the planet.
  • Dungeons & Dragons:
    • Baldur's Gate II has the Underdark, a vast subterranean labyrinth covering much of the world.
    • Baldur's Gate III: At the end of the first act, the party heads through the upper layers of the Underdark to reach the next story destination. The path winds through dark caverns hanging above deeper, inaccessible abysses, lit by the faint light of huge glowing mushrooms and home to feuding factions of mushroom people and duergar.
  • Dwarf Fortress has no fewer than three levels of world-spanning underground cavern systems, home of mushroom forests, tribes of animal people and some very weird and hostile monsters, with each layer getting progressively nastier than the last. This is also where you will find Forgotten Beasts, unique, randomly generated, and very powerful and dangerous immortal monsters. Dig down past all three of these, and you'll find a great sea of magma and semi-molten rock. And beneath that, if you can contrive to get at it, is Hell, which can be surprisingly easy to reach, provided you can find the right veins of mineable rock and have a decidedly casual attitude towards being slaughtered by sheep monsters made of ice.
  • EarthBound (1994): Lost Underworld is a massive cavern that is home to a tribe of Tendas and Living Dinosaurs. It's also the site of both the final Your Sanctuary location, Fire Spring, and the final area of the game, the Cave of the Past.
  • Elden Ring:
    • The Lands Between is full of hidden catacombs, caves, and crystal mines the Tarnished can stumble upon, most of them even having a Boss at the bottom of it. Furthermore, some places are their own little world beneath the earth, such as Nokron, the Eternal City, which has a sky full of stars (actually glowing glintstone ores) despite being underground.
    • The Siofra and Ainsel Rivers that are connected to their respective Eternal Cities, Nokron and Nokstella, also expand on the explorable undergound sections of the world. Going deeper into the Ainsel River leads you down to the Lake of Rot, which is implied to be the sealing ground of the Scarlet Rot's outer god.
    • The Subterranean Shunning-Grounds, which serve as the sewers to the capital city of Leyndell, can lead even further underground, to the point where a secret passageway through the Frenzied Flame Proscription, itself a hidden area deep underneath the bottom of the sewers, leads to the Deeproot Depths, the source of the aforementioned rivers and the beginning of the Erdtree's network of roots spreading through the Lands Between, on top of being home to a third, nameless Eternal City.
  • The Elder Scrolls V: Skyrim: Blackreach is a giant cave the size of a small country, housing forests of glowing mushrooms and an ancient Dwemer city complex. This, as well as other Dwemer ruins throughout Skyrim, have largely been taken over by the Falmer, a race of subterranean goblin-like creatures who were once the graceful Snow Elves but were taken in and twisted by the Dwemer when their race was threatened by the invading Nords.
  • EverQuest II: The "Terror of Thalumbra" expansion introduces players to the titular Thalumbra region, a massive ecosystem fully encompassed within an almost endless cave. The city of Maldura is inhabited by the Dhalgar and Gnemlins — cousin races to the Dwarves and Gnomes. The Black Sun Sea expands beyond the horizon to the east and southeast of the map. The Glaufaye are willing to explain lore of ancient gods who were killed long before the current pantheon came to be, and the Mindflayer-like Abherrants thrive in an ancient temple found even deeper below Thalumbra itself.
  • Exile takes place in a gargantuan network of underground caverns, stretching for several kilometers in every direction, and varying from huge caves big enough to have their own weather systems, to twisting little mazes of passages, all alike. As the name of the game suggests, the people who live there didn't choose to do so, and, sure enough, the place is littered with fungus, lava, and plenty of gloom.
  • Fallen London: The Neath is a vast cavern beneath the Earth's surface, well away from the Sun's light and laws. London was transported under the earth as part of three terms of a Deal with the Devil on the part of Queen Victoria to save Prince Albert's life, and is now just down the river from Hell. Sunless Sea explores the Unterzee, the vast underground ocean that surrounds London, whose islands are home to strange cultures and ancient horrors and whose inky waters teem with terrible monsters.
  • Genshin Impact: The Chasm in western Liyue. At the closest level to the surface, the cavern is only filled with miners, mining equipments, and the occasional Treasure Hoarders, but going deeper will reveal larger caverns, more wonders and dangers.
  • Hollow Knight: Hallownest is an enormous abandoned subterranean empire, filled with beautiful vistas and infected horrors, that the player spends the game exploring.
  • Kirby Super Star features a Metroidvania-type game called "The Great Cave Offensive", in which Kirby has accidentally fallen down a hole and found himself in an underground world filled with treasure. Now he has to find a way out while grabbing as much loot as he can along the way.
  • Knights of Ambrose: There's an additional world map underground known as the Ambrose Underworld, which contains several dungeons, a vast cave network, Tiamat's lair, and seas of lava.
  • The Legend of Zelda:
    • The Legend of Zelda: Oracle of Seasons: Subrosia is a volcanic subterranean realm located underneath Holodrum that is only accessible through the use of Portals.
    • The Legend of Zelda: Tears of the Kingdom:
      • Some of the larger surface-world cave systems can be very extensive, forming localized mazes of branching tunnels, side chambers and underground lakes and rivers. These are home to glowing plants and animals, camps of apelike Horriblins, and rich mineral deposits.
      • The Depths are a vast subterranean area spanning most of Hyrule's underbelly that serves as a Dark World. They're home to a much more alien ecosystem than the surface caves, characterized by high monster populations and forests of trees with strange feather-like branches, immense mushrooms, and gigantic fern-like plants. Initially, the Depths are pitch black save for those places you've lit up with Brightbloom seeds, only fully lighting up as you activate Lightroots. There are also lots of monster-controlled mines there that are the main source of Zonaite. This is where Ganondorf was sealed away in the distant past; all the Chasms opening up the Depths to Hyrule's surface are the result of his Gloom bursting forth upon his awakening at the story's beginning.
  • Minecraft:
    • Natural caves are procedurally generated like every other landform, and get bigger and more extensive the deeper down you go, together with an increasing likelihood of separate caves having connections with each other. Near-surface caves are fairly small and modest, but the ones near the bottom of the game world can get very big indeed, and are almost always interlinked, resulting in huge underground networks. Naturally, these caves are pitch dark unless you light them up with torches, and as such are crawling with hostile monstersnote . Underground lakes and rivers are also common, as are lakes and waterfalls of lava that provide the only natural light down there.
    • The 1.17 and 1.18 updates, collectively referred to as the Caves & Cliffs update, extensively expand on and add to the caves of Minecraft. In addition to doubling world depth in order to create extra space for caves, an update to the cave generation programming results in much more extensive cave systems generating as a mixture of winding labyrinths of long, thin tunnels interspersed with immense caverns. Aquifers can also create large underground bodies of water and lava, the former home to schools of glowing squid. Three new cave biomes also generate — dripstone caves, filled with forests of speleothems; lush caves, filled with plant growth and bodies of water and lit by vines with glowing berries; and the deep dark, an area found in deepest areas of the world that's covered in dark Meat Moss, filled with ancient ruins, and haunted by a terrible monster.
  • Poacher: The subterranean world that Derek Badger finds himself trapped in does not only have towering cities inhabited by disembodied spirits, but also beaches and forests, and even a jungle.
  • In Roots of Pacha, the Caves are a complex underground network that interconnects the Forest, the Beach, and the Savannah. They contain rocks and gems that are mined for resources, and some rocks emit magic energy that opens pathways deeper into the network. Sapient, magical glyptodons make their home here, and they're training to become Totem Animals like Owl, Monkey, and Bear, who are guardians that bestow the protagonist magical powers when they complete their challenges. The glyptodons also use a small Tunnel Network that the protagonist can use to fast travel to various rooms in the Caves or the areas on the surface.
  • In Tavern Talk, the Plane of Ur is a labyrinthine cave system that sprawls beneath the roots of the Aether Tree. It has all sorts of magical gems and ores buried inside, and anyone lucky to stumble into the Plane is at a loss of words on how to describe it.
  • Terraria has an extensive cavern system honeycombing the world underground, consisting of three layers. The Underground is the first layer beneath the surface, and contains mostly of soil tunnels home to slimes and giant worms; caves here are relatively small and mostly isolated from one another. Beneath that is the Cavern layer, which makes up the largest vertical portion of the map, and where caverns are much larger and form a consistently interlinked network. More dangerous creatures are found here, alongside exotic biomes such as frozen caves, areas overgrown with lush vegetation, and forests of giant glowing blue mushrooms. Occasionally, you'll break into a cavern containing a mostly-intact cabin, but there are also Granite Caves, mini-biomes made of dark stone that often contain several empty buildings, as well as nests full of Giant Spiders. Then you reach the Underworld, a lake of lava, dotted with spires of obsidian and Hellstone, stretching the entirety of the game map.
  • Touhou Project:
    • Touhou Chireiden ~ Subterranean Animism introduces a society of "hated" youkai living underground. Surface youkai are forbidden from entering their realm due to an ancient treaty, but the opposite is not true. This realm includes the former site of Hell — it was relocated due to overcrowding, but some of the workers and evil spirits are still around.
    • Touhou Gouyoku Ibun ~ Sunken Fossil World: The story has its characters traveling down to Former Hell, only to visit a place even lower than Kanako's nuclear reactor: the Hell of Blood Pools, where the congealed emotions of countless organisms turn into cursed petroleum as the literal "blood of the Earth" and titular submerged hell of sunken sorrow.
  • Ultima: In the fifth game, deep below the surface of Britannia is the Underworld, a wild neverland of linked caverns and river rapids that collapses at the game's end. In Ultima VI you can proceed through the ruins and reach the other side of the flat earth.
  • Undertale: The Underground is a cavernous area beneath Mt. Ebott that is large enough to have various biomes, such as a Lethal Lava Land, a Slippy-Slidey Ice World which somehow has snowfall, as well as a Creepy Cave that's noticeably narrower and filled with Glowing Flora.
  • Warcraft:
    • Azjol-Nerub, the capital of the Nerubian civilization, is a sprawling underground labyrinth that stretches across much of the continent of Northrend. The part where the Nerubians used to live is called the Upper Kingdom, and below that is the Old Kingdom, which consists of even deeper tunnels populated by nameless horrors and one of the Old Gods, which was confirmed to be Yogg-Saron in the Wrath of the Lich King expansion.
    • World of Warcraft: Khaz Algar, the main setting of The War Within, is a subterranean land beneath the oceans of southewestern Azeroth. It is primarily divided between the cave system of the Ringing Deeps, where a large relic population of Earthen, the elemental ancestors of the dwarves, live; Hallowfall, a huge, lush open cavern lit by a giant glowing crystal in its ceiling and bordering a subterranean sea; and Azj-Kahet, the deepest layer, home to an ancient and hostile kingdom of Nerubians. Also living down here is a large population of the blind, mole-like niffen.

Webcomics 

  • A Beginner's Guide to the End of the Universe: After reaching the Dark Star, the Everyman enters a vast system of lightless caverns that wind on for miles. He encounters nothing there besides a single Giant Spider and, eventually, the Singularity's hidden lair.
  • Aurora (2019): The inside of the planet is riddled with caverns and tunnels, the remains of the Stone Primordial's circulatory system. They're also full of creatures that were once regular people or animals that were corrupted by a mysterious force that is implied to be the Void Dragon's power.
  • Drowtales: The bulk of Drow civilization consists of a handful of large subterranean city-states within large caverns in an extensive underworld, separated by large stretches of stony wilderness home to isolated outposts, driders, degenerate Drow Mole Men, and the few Dwarf nations that the Drow didn't wipe out. It was created alongside the sky world during the first of two apocalyptic wars waged by ancient elves when a substantial piece was broken from one of the moons and fell to earth, with the underworld being, in essence, the cracks in the world caused by the impact. At least one section, the Nidavellir, is open to the skynote  but filled with a sea of fog. There's also a second underworld under a different continent from the main one, ruled by its own Drow nation, Hel. It's not a nice place.
  • Mare Internum: Most of the story is set in a complex system of partly flooded volcanic caverns beneath Mars' surface, which are still inhabited by a variety of Martian organisms and Organic Technology.
  • Sam & Fuzzy: The Underground is a massive, world-spanning set of caves complete with underground cities, roads and small communities. It's home to practically everything supernatural that humanity has ever thought was real, mixed with some sci-fi elements like artificial robots.

Web Original 

  • Codex Inversus: The Principality of Dis is built atop the ruins of Tartarus, one of the primary areas of Hell before the Collapse. In the World Before, it was a cosmic prison of labyrinthine, shifting tunnels that held dangerous rebels against the cosmic order; in the modern day, its ruin is a sprawling, mostly unmapped cave system prone to shifts as new passages open and older ones collapse. Its depths are home to groves of giant fungi, bizarre ecosystems supported by natural concentrations of mana, treasure troves of pre-Collapse wealth, and the remains of ancient giants, demigods and sinners, all of which draw explorers to plumb the abysses — but, just as often, terrible monsters emerge from the unknown depths of Tartarus to ravage the surface world.
  • The Crew of the Copper-Colored Cupids: The Worlds Below, a network of magical caves beneath the Prime Earth, which include such things as the cavern where Loki is bound, appear in the story The Winter Quests.
  • Orion's Arm: Many worlds have extensive underworlds, either artificial or natural, which are often populated by fairly large societies.
    • Cenote was covered by extensive layers of calcareous stone by a now-extinct biosphere, and over millions of years its waters eroded immense systems of flooded caverns larger and more extensive than any other natural cave system in the known galaxy. Its original colonists opted to take aquatic forms to best explore this flooded underworld and spent thousands of years mapping out its global cave systems. These included caves large enough to build cities in, as well as some dry caves filled with extremely large stalactites, calcite crystals and similar formations.
    • Mercury is a particularly notable example; over its long history, extensive mining activities left global mazes of tunnels and caves, some very large, running through its crust and upper mantle, while later megaprojects added two layers of habitable space above its original surface. Beneath the planet's uppermost surface, modeled to resemble the original Mercurian environment and mostly home to mobile cities, the depths are currently divided into five layers:
      • The topmost, Undervale, is a continuous layer pressurized and terraformed to resemble Mars, and mostly inhabited by colonists from the red planet itself and some others adapted to similar conditions.
      • Next down is Mercury's original surface, called Elderdasht. This has itself been returned to the planet's original state, with the exception of the original settlers' cities which are now encased in Earth-like environments.
      • Below comes Wotum, an agricultural layers with light arrays and a simulated sky on its roof and artificial rain. It's used to grow food for the rest of the planet through a combination of natural, genetically engineered, and nanotech crops.
      • Khazad-dum is a series of distinct, regularly-shaped caverns extending in a geometric pattern beneath the whole planet. They're kept unlit and are home to a large population of sapient robots that constantly demolish and rebuild the empty cityscapes that fill it, alongside small communities of biological sapients that inhabit lighted islands in the middle of the dark emptiness.
      • The lowest layer, Bism, consist of a number of large, interconnected cavern systems expanded from the ones dug by original mining operations. These are kept lit by luminous plants or fungi, or by lakes filled with bioluminescent microorganisms. Its settlements include Stalactitopolis, a city hanging from the ceiling of a large cavern and above one of the glowing lakes.
  • SCP Foundation: SCP-2622 claims to be from a subterranean world straight out of pulp fiction, home to a civilization of Mole Men, evil reptilian people, caverns full of prehistoric life, and giant caverns filled with volcanic activity or vast crystalline fields. He's making everything up.

Western Animation 

  • The Dragon Prince: Umber Tor, home of Rex Igneous, the archdragon of Earth, is a series of underground chambers beneath a mountain that are meant to test those who seek the dragon, such as a crystal labyrinth, the "bridge through darkness" and a cave of lava called Lava Land.
  • Dragons: The Nine Realms: The Hidden World is a vast realm beneath the Earth's surface — it's accessed from a caldera in the Viking lands in the original movies, but is opened up to the world due to the creation of a giant fissure in the series. Being based on the World Tree Yggdrasil from Norse Mythology, it consists of nine gigantic caves called "realms", some of them with their own caves and tunnels. The nine realms are connected by an intricate network of tunnels, some natural, others made by dragons. Among the realms, there are the Fire Realm, which is covered with volcanoes and a floor of lava, the Ice Realm, a section of the Hidden World constantly covered in snow and slippery ice, the King's Realm, a mountainous region, and the Nature Realm, which has an abundance of plants as well as a giant underground ocean, just to name a few. In-universe, this biodiversity is explained to be possible because of the Dragoncite, a Fantasy Metals that creates oxygen when being blasted by dragon fire.
  • My Little Pony:
    • My Little Pony 'n Friends: "The Quest of the Princess Ponies": The Kingdom of the Lava Demons is a large network of caverns and tunnels beneath the Crystal Desert, most of them containing pools and rivers of molten rock.
    • My Little Pony: Friendship Is Magic:
      • "A Dog and Pony Show": The Diamond Dogs are a group of subterranean dog-men who inhabit a labyrinthine system of tunnels and large caves.
      • In later seasons, Rarity usually goes gem hunting in a long tunnel studded with giant gems. In "Rock Solid Friendship", it's revealed that there's a large cave filled with plant life and lit by a cluster of giant Glowing Gems connecting to it, where Maud sets up her home. In "Uncommon Bond", it's further shown that this cave connects to several mazelike tunnels and beyond that the cave of the mirror pool, ultimately forming a fairly extensive system of interconnected caves running beneath the outskirts of Ponyville and the Everfree Forest.
  • Os Under-Undergrounds is majorly set in the Underground, an underworld civilization full of mutant Mole Men. The story starts when Hector, the Token Human, ends up falling down a drain and ending up there.

Real Life 

  • It was a very widespread belief among Greek and Roman philosophers and naturalists that there were great caverns, abysses and channels running throughout the depths of the earth, containing rivers, swampy morasses, and great lakes and seas. This was in part an attempt to explain how rivers could spring out of seemingly solid rock and where they went when they vanished into caves and partly to explain the origins of earthquakes and volcanic eruptions — most accounts of these events tended to assume that they were caused by violent underground storms, floods or turbulences shaking the foundations of the earth, masses of gases or underground fires bursting through the surface, or caverns collapsing into themselves.

Alternative Title(s): Fantastic Underground