The Unknown Cave is a secret location in Final Fantasy V. It is a one-room cave with no monsters, and only one inhabitant: Mr. Clio. He can show the player their various stats, including number of battles fought, number of monsters slain, average Experience, percent of treasures looted, and number of times the game was saved.
The Unknown Cave is one of three locations in the Merged World not to be indicated on the map with a blinking star (the other two being Phantom Village and the Sealed Temple), so it can be frustrating to locate at first. To find it, the player must land the airship in the small sea directly south of Karnak and submerge. On the ocean floor, there will be a purple-colored rock with a hole in the middle. This is the entrance to the Unknown Cave.