A cave on Risa
A cave is a hollow passage underground, usually with an opening to the surface. Caves can be both natural or artificial, and are found on most every landmass in the galaxy.
Caves, or caverns, can make excellent hiding locations as the rocks and ores in the walls can stop sensors from penetrating them. They also can make excellent shelters from the outside environments. For example, Data, Riker, and Doctor Crusher led the colonists on Melona IV to shelter in caves to escape the destruction of the Crystalline Entity. (TNG: "Angel One", "Silicon Avatar"; VOY: "Parturition")
Native Americans often used caves as shelters during their vision quests. (VOY: "The Fight")
In 2285, it was revealed that the Starfleet Corps of Engineers had spent ten months excavating a massive cave system on Regula, while a matrix to terraform the caves needed only a day. (Star Trek II: The Wrath of Khan)
In 2351, a ten year old Julian Bashir and his father, Richard were trapped in a cave on Invernia II by a sand storm. (DS9: "Melora")
In 2373, the crew of the USS Voyager discovered a large cave-dwelling creature that ate humanoids, the Hanonian land eel. (VOY: "Basics, Part II")