Belly of Unagi

Belly of Unagi is allegedly a hidden course found in Super Mario 64's Jolly Roger Bay course. It is most commonly accessed by being eaten by Unagi the Eel on any star in which they hide in their tunnel. By aligning Mario in a certain way in front of Unagi, Unagi will swallow Mario without actually killing him, warping him to the secret course. The course shares it's music with Hazy Maze Cave and Wet-Dry World.

Geography
When warping to Belly of Unagi, Mario will initially spawn in a large area resembling a mouth. Behind Mario is a pair of large teeth that, when broken, can allow Mario to go through them, warping him back to Jolly Roger Bay. The platform Mario stands on is a large tongue, with multiple Goombas, coins, and boxes found around the area. By going down Unagi's throat, Mario will find himself in an underwater area filled with coins and Bubs swimming around.

By swimming deeper down Unagi's throat, Mario will find himself in a large area filed with stomach acid, which acts like lava. Parts of a broken pirate ship can be found floating in the acid, which can be used as platforms which slowly sink down when Mario is standing on it. Boos can also be seen, alongside Keronpa Balls. In addition to these enemies, a slightly modified version of the unused Blargg enemy can be seen in the acid, being colored green instead of red. Certain parts of the broken ship can be entered by Mario. Inside the destroyed ships are large areas filled with Boos and Bob-ombs. Certain areas also have Metal Cap crates and coins.

Missions

 * 1) TOP OF THE BROKEN SHIP
 * 2) RED COINS IN THE STOMACH AREA
 * 3) BREAKING THE GOLDEN TOOTH
 * 4) METAL MARIO BOUNCES AROUND
 * 5) BOO BUSTIN' IN THE TUMMY
 * 6) THE BIG BLARGG
 * 7) 100 COINS

TOP OF THE BROKEN SHIP
The first mission Mario must complete when entering Belly of Unagi. Here, Mario must travel to the stomach and jump across the floating parts of the ship. From there, Mario must climb up the tallest part either by wall-jumping from the walls of the stomach or by entering the ship and reaching the top. By reaching the top, Mario will find a star which he can collect.

RED COINS IN THE STOMACH AREA
A basic red coin star. Mario must collect all red coins floating on parts of the ship in the stomach in order to summon the star.

BREAKING THE GOLDEN TOOTH
One of Unagi's teeth is colored a bright yellow. By carrying one of the boxes on Unagi's tongue and throwing it at the golden tooth, it will shatter, revealing a Star which Mario can collect.

METAL MARIO BOUNCES AROUND
In order to obtain this star, Mario must first transform into Metal Mario by going inside one of the destroyed ships and finding the metal cap. From there, Metal Mario must bounce on top of the stomach acid in order to reach a ledge on one of the walls of the stomach where a Star is kept. Alternatively, Mario can wall-jump up to the ledge to obtain it quicker.

BOO BUSTIN' IN THE TUMMY
Mario must simply defeat all of the boos in the stomach. From there, a star will be summoned on one of the floating pieces of a ship.

THE BIG BLARGG
Mario must first obtain the Metal Cap using the same method as in Metal Mario Bounces Around. From there, Mario must bounce across the stomach acid to a small hole, in which he will find himself in a large area with a massive Blarrg. Mario must then travel across the area while avoiding the Blarrg's rush attacks in order to find more Metal Caps, in which he can ground-pound the Blarrg three times in order to defeat it, leaving behind a star.