Nipper Waterfalls

Nipper Waterfalls is the name given to a supposed level found in some personalized copies of Super Mario 64. It is generally described as a cave behind a waterfall with several Nippers, which as the name suggests is this level's signature enemy. It is generally a smaller level instead of a full course. It is supposedly accessed by using the cannon in the Castle Grounds to launch behind the waterfall, sometimes having to defeat the Hammer Bros. boss fight beforehand.

Description
As previously stated, Nipper Waterfalls is described as a cave level behind a waterfall, where the player usually spawns. It is commonly described as being filled with water, having several rivers, lakes and ponds, likely due to the waterfall. It is generally rather linear, with several Nippers, Piranha Plants, and Goombas, as well as Bubs underwater. Notably, Bob-omb Buddies have also been reported, usually complaining about a Nipper infestation.

Stars
The most common mission is completed by defeating all of the Nippers in the level. Other missions include beating the Hammer Bros., who in this scenario can generally be found somewhere in the level instead of being fought before the level is entered, and collecting 8 red coins. Some also say that Dorrie can be found in the level, and like Hazy Maze Cave, must be used to reach an island with a Power Star.

Theorizing
It is believed that if this level was designed by the development team of Super Mario 64, Nipper Waterfalls was removed by Nintendo due to being too hard to locate thanks to it's rather obscure location. It also may have been removed due to it ruining the 100% completion reward of locating Yoshi, as you would have to use the cannon to find this level too, so you could find them before having every Power Star. Of course, none of this stops the Personalization A.I. from reimplementing it into the game.

Notably, Nippers were discovered to be legitimately unused enemies in the source code of Super Mario 64, though they appear to have been planned for Bob-omb Battlefield. This could mean they were planned for Bob-omb Battlefield before Nipper Waterfalls was created, or they were going to be moved to Bob-omb Battlefield after Nipper Waterfalls was removed, but for whatever reason were also scrapped. Alternatively, Nipper Waterfalls could have been created via Dynamic Level Arrangement by the Personalization A.I. to give Nippers a place to appear, mixing them together with some legitimately unused waterfall area and perhaps the Hammer Bros.