Cavern of the Unagi

The Cavern of the Unagi, also known as Unagi Caves, is a supposedly cut area from Super Mario 64. Named after the eel enemy Unagi, it was intended to be entered through Unagi's only appearance in Jolly Roger Bay, through the alcove in which it inhabits in every mission aside the first. While blocked off in normal gameplay, it was intended that this area would be opened up after Unagi left so that the player could enter this hidden area.

While the area was removed from the most circulated release of the game, some accounts claim that the area returned within their personalised copies of the game.

Geography
The Cavern of the Unagi continues where the entrance to it left off, within a winding tunnel that leads into a room where the player can get some air. This room is reminiscent of the shore of Jolly Roger Bay, although is barren of any enemies or objects aside from the stray boxes of the pirate ship. Some explorative players who have encountered this room claim that there are cracks within the wall that they could see the sky through.

Moving deeper into the area leads down a winding path on an elevated slide, leading into the water and down into the last room. This room is filled with an unusual black goop liquid unseen within Super Mario 64, resting at the bottom of a pool despite being underwater. An Unagi patrols this room, making an active effort to chase Mario down in the attempt to eat him.

Counted as one of the Castle's Secret Stars, a star can be located within this last room within a cage. The players that found this room described having to lure the Unagi into the chain which holds the cage, causing it to drop and break open, freeing the star.

Belly of the Moray
As many recounted their experiences with this bizarre unseen area within Super Mario 64, an anonymous user claiming to be a former playtester at Nintendo of America recounted their experience with this level, or rather a much earlier form of this level. Known at the time as "ウツボの腹" ("Belly of the Moray"), the level was entered instead when the player was eaten by the much more aggressive Unagi.

Forgoing the tunnel people talked about, the stage started within the shore, or rather the mouth as the playtester thought. The cracks in the room made the walls seem more like teeth, and the texturing had a stylised yet disturbing flesh-like texture. The level continued with this theming, as well as the absence of water. The pool of black goop that filled the last room was instead of a green lava, likely intended to be representative of stomach acid. Instead of facing an Unagi, the player instead had to platform across several dishevelled and bobbing boxes that led up to a star.

Supposedly, many playtesters condemned this level for its bizarre imagery and difficult platforming. This has led many to assume that the developers tried to salvage the level by turning it into the Cavern of the Unagi, before eventually scrapping it all together. It can be inferred that the platforming section may have been repurposed into the interior of the sunken ship.