Behind the Waterfall

Behind the Waterfall refers to an event reported in Super Mario 64 that's about the waterfall located in the top-left part of the outside of Princess Peach's Castle. The event is about a secret area that's behind the waterfall, which was alleged to be unlocked after obtaining all 120 Power Stars in the normal game. This event has been stated to be an alleged method to unlock Luigi in the game through a boss fight.

The Behind the Waterfall occurrence was reported on a Neoseeker forum pertaining to rumors about Super Mario 64. The link to this forum can be found here: https://www.neoseeker.com/forums/1191/t861112-super-mario64-rumors/

Information
The prerequisite for enabling the event would be that Mario has to collect all 120 stars in the base game, in which the Wing Cap and cannon outside of the Castle would be usable. Using the cannon and Wing Cap, Mario must go to the waterfall area. A 120-star door would now appear in the waterfall, where Mario can open it with all the stars he has.

The door would lead to an area that contains an alleged Hammer Bro. boss. The area itself is stated to be a "tube", which is most likely inside of a Warp Pipe. Defeating the boss would have a new cutscene occur in the game. This cutscene would start off with Mario at the bridge area of the castle with Princess Peach, like in the ending cutscene. However, Bowser then appears and kidnaps both. Luigi, being in the outsides of the castle, was given the Poltergust 3000 by Toad to save the two. After this cutscene, Luigi would allegedly be unlocked as a playable character.

Poltergust 3000
The Poltergust 3000 is a vacuum like machine that made its first official appearance in Luigi's Mansion, 5 years after Super Mario 64 was released. As such, it seems anachronistic that Super Mario 64 would have the Poltergust 3000 in it. However, development on Luigi's Mansion started on the Nintendo 64, the same console that Super Mario 64 was on. The Poltergust 3000 could have been a leftover from development on Super Mario 64, when multiplayer was supposed to be in the game. It could be possible that the Poltergust 3000 was reused for Luigi's Mansion.

It is unknown if the controls of Luigi using the Poltergust 3000 were similar to in Luigi's Mansion, or that Luigi would have a different control scheme with it. It is unlikely that the former would be the case, considering the controls of Luigi's Mansion wouldn't translate well with Super Mario 64.

Hammer Bro.
The boss fight of Hammer Bro. that's reported to be in the Behind the Waterfall effect seems to be odd. Considering that Hammer Bros. were usually an enemy found in the Mario games as of this time, it being a boss now would be a strange choice. It is possible that the choice for Hammer Bro. being a boss in the game was done as a way to fit the object limit of the game. If the Hammer Bro. boss would throw hammers (most likely), the hammers each would be considered separate objects. It also could have been done as a way to balance the game, as Hammer Bros. are hard enemies to avoid and defeat. Due to Super Mario 64 being the first 3D Mario game to be worked on, it would be understandable that Hammer Bros. wouldn't be normal enemies in the game if they were in it.

Personalisation A.I
The Behind the Waterfall event would be considered to be a result of the Personalisation A.I making changes towards Super Mario 64. This could have been done as a way to make a method to unlock Luigi in the game; taking the unused feature of Luigi being playable and using it. Using this in mind, it can be assumed that the tube area and the Hammer Bro. boss fight were unused features in the game.

Strangely, even though the July 25th 2020 leak had unveiled an unused Luigi model and unused areas in the game, neither the tube area, the Poltergust 3000, nor the Hammer Bro. boss fight described in the event appear in the game. It is not out of the realm of possibility that the event itself was unused, considering the leak didn't show all of the unused/beta content from Super Mario 64.

Trivia

 * The event of unlocking Luigi at the end of Mario's game after collecting 120 stars would be later used in Super Mario Galaxy. It is unknown if this was inspired from the Behind the Waterfall event.