The Crescent Mountain Cave (三日月山の洞窟, Mikazuki-yama no Dōkutsu?) is a location in Final Fantasy VI. It only appears in the World of Balance on the Veldt, and serves as the entrance to the Serpent Trench. It is where Gau keeps his "shiny thing", which is a diving helmet.
Sabin, Cyan, and Gau pass through here. Gau pulls several tricks on Sabin (some of which are harmless, but one causes the player to lose some gil), but he eventually shows them the diving helmet, and the three jump into the Serpent Trench and head to Nikeah. There are no enemies inside.