A vast cavern hollowed out within the lifeless Regula planetoid and transformed into a lush habitat as the second-stage test of Project Genesis.
The Genesis cave is seen in Star Trek II: The Wrath of Khan. Carol Marcus's team first spent months tunnelling storerooms into the barren planetoid beneath the Regula I laboratory, and then used the Genesis process to turn a huge chamber deep underground into a self-sustaining world of vegetation, water and artificial daylight.
When Khan attacks the station, Carol, David Marcus and a few colleagues hide there with the Genesis torpedo. Kirk's party follows, and Khan beams the device away and leaves them apparently entombed.
In the cave Kirk talks with Carol about their son and admits how old and worn he feels. The marooning is brief, since Kirk and Spock had arranged a coded schedule that lets the Enterprise retrieve the party within hours.