Catfish's Maw

Catfish's Maw is the fifth dungeon Link's Awakening. The entrance of this dungeon, which looks like a giant, blue catfish, is located within Martha's Bay. The entire entrance is surrounded by rocks, and thus cannot be normally accessed by swimming on the surface alone. Link will need to dive down a certain spot on the left side of the rocks and swim through a hidden, underwater tunnel to this dungeon. Since Link needs the Flippers to swim and dive, it's possible for him to enter Catfish's Maw before even clearing the dungeon before it, Angler's Tunnel. The interior of the dungeon is mostly sandy grounds with brown walls. There also seems to be a lot of purple crystals growing in certain rooms, which can be destroyed by Link's sword. Despite this dungeon being in the middle of a bay, it appears to be actually dry inside, with only a few rooms having swallow water and only three rooms being flooded (Two of them being flooded side-view rooms.)

The main tool of this dungeon is the Hookshot, which is necessary to access certain rooms and needed to defeat the boss, Slime Eel who guards the fifth Instrument of the Siren, the Wind Marimba.

This dungeon is notable that it has at least five mini-boss battles. The first four all involve Master Stalfos who guards the Hookshot object, but needs to be defeated four times. Once defeated, he will move into another room and must be searched for in order to battle again. All of the rooms in which Link battles him can easily be identified by the skull-shaped tiling on the room's grounds. Each of these four rooms also has an unmovable block or blocks, the number of blocks points out when Master Stalfos would appear there. For example, the first room Master Stalfos is in has a single block, but once defeated he will move into the room with two blocks and so on. The other mini-boss are the Gohmas, who can only be reached once Link has the Hookshot and are considered the 'true' mini-bosses of this dungeon.

When looked on a map, the dungeon is in the shape of what seems to be an eel, likely a reference to this dungeon's boss, Slime Eel who is indeed very eel-like in appearance. The name of this dungeon is simply a reference that the exterior is in the shape of a giant catfish, and that the entrance is the 'maw of the catfish'.

Trivia
Catfish's Maw can be looked at as an early prototype or concept for the future dungeons: Inside Jabu-Jabu's Belly and Jabu Jabu's Belly. Although Catfish's Maw isn't considered to be a 'living thing', but more or less where its exterior is styled in the shape of a catfish.