Tunnel
A map of tunnels
An artificial tunnel on Velara III
A tunnel was an underground passageway.
The Novans on Terra Nova lived for three generations in an underground network of tunnels and caves. In 2151, several tunnels collapsed when Shuttlepod 1 crashed beneath the surface. (ENT: "Terra Nova")
The catacombs beneath the Vulcan monastery at P'Jem were made of several tunnels. (ENT: "The Andorian Incident")
While at Janus VI in the 2260s, Captain James T. Kirk and his first and science officer Spock encountered several tunnels created by a Horta, who was capable of moving through solid rock as easily as Humanoids could move through air. (TOS: "The Devil in the Dark")
Tunnels once formed part of the Phylosians' home. (TAS: "The Infinite Vulcan")
In 2270, the crew of the USS Ariel tried tunneling to escape the Lactra VII zoo, but were unsuccessful. (TAS: "The Eye of the Beholder")
The terraforming science team on Velara III created artificial tunnels to start their project by using a laser drill. In one of their tunnels the away team of the USS Enterprise-D discovered the "microbrain" lifeform. (TNG: "Home Soil")