Cave tiles are mountains with usually just one, but occasionally more caves. The cave is on the east side of the tile; in other words, if you enter the tile facing west, you will start in front of the cave.
Surrounded by rock walls, caves are pitch black inside. In order to see, you must either build a fire or wield a torch. You can build a floor inside the cave without the need for walls, making the existence of cave houses possible. Caves are generally not more than 20 tiles large. Rocks which block your way can be removed with what is technically an exploit; simply build two fences next to the rock, and then build and deconstruct a door on the rock.
Prior to version 3.12b, patch 2, caves often contained animals and treasure. After the patch, this is very rarely the case. Animals may appear on cave tiles.
More than once a Njerpez warrior has been encountered lurking near or in a cave.