In The Legend of Zelda: Link's Awakening, the first dungeon is Tail Cave.
Tail Cave is located on the southern coast of Koholint Island, near where Link's journey begins after being shipwrecked. It is the first of the game's eight main dungeons and introduces the player to the standard Zelda dungeon structure: exploring rooms, collecting keys, finding a dungeon map and compass, obtaining a major item, and defeating a boss.
The dungeon's name comes from its distinctive tail-shaped layout and its connection to the island's animal-like imagery. Inside, Link faces early enemies and puzzles before confronting Moldorm, a large, segmented worm-like boss. Defeating Moldorm earns Link the Full Moon Cello, the first of the instruments needed to awaken the Wind Fish.
Tail Cave is often confused with later dungeons because Link's Awakening has several memorable names, including Bottle Grotto, Key Cavern, and Eagle's Tower. Those are not the opening dungeon: Bottle Grotto is the second, Key Cavern is the third, and Eagle's Tower is the seventh. The original Game Boy release and later versions retain Tail Cave as the first main dungeon.