Princess Peach's Castle/Bowser's Domain

A large area supposedly beneath Princess Peach's Castle. Bowser's Domain was first documented by youtuber Marionova64 in his videos Breaking the Barrier and Subterranean Terror. Bowser's Domain is a secret hidden base supposedly ruled by Bowser that can be accessed via falling down a second trapdoor in the hall that leads to Bowser in the Dark World, although the second trapdoor can't be accessed in most copies of Super Mario 64, it still seems to be present in atleast the uploader's.

Bowser's Domain also contains a strange version of the Bowser Room.

Layout
Bowser's Domain presents itself as a large area of the castle accessed via falling down a second trapdoor in the hall that leads to Bowser in the Dark World. After this, Mario will land in a large hexagonal room, right in the center of a carpet similar to the Foyer's. This room has several doors and paintings hung up on the walls, two of these paintings are Bob Omb Battlefield's and Whomp's Fortress', there is, however; a third painting of a Goomba in a green field, with a door underneath. There are several other doors, too. Behind the player's landing spot is a large slope with a red carpet texture and a sign adverthim to not open the door at the end of the slope (Presumably written by Bowser). The walls next to the slope have a different texture to those in the main room, as they seem to resemble a star night sky similar to that of the painting in the second floor. At the very end of the slope is a door using textures found in Ocarina of Time and Wet-Dry World.

After opening the door, the player will enter a smaller area similar to the Castle Courtyard, with some windows, a large fountain in the center of it, and two more doors with the same texture. However, most notably, a painting using the Wet Dry World skybox can be found directly in front of the fountain, the painting is accessible, although it is unknown where it takes the player to. The door directly infront of the player's landing spot leads to a strange room similar to the entrance of the Basement, with two paintings: Wet Dry World's (JAP version) and Lethal Lava Land. There's also a tunnel-like entrance extremely similar to the Basement. The tunnel leads to an extremely confusing complex of halls, and a sign written by a Koopa worker, documenting their work and complaining about how King Bowser treats his workers. This strange complex of rooms also has several doors, particularly, one of these doors leads to a three-way room, with several strange, never-seen paintings. The center way of this room takes the player to a small room, with a pit, a sign, and a door Another one leads to the Koopa Library, full of shelves and signs; and weirdly enough, a large Cap Switch, with a sign warning the workers to not interact with it. There's also a door with a Bowser painting hanging above it, and a sign warning us not to open it, as it leads to the infamous Bowser Room.

Notes.

 * The music used in all zones of Bowser's Domain (except the Bowser Room) is a slightly modified version of the Castle's theme.
 * The Bowser Room uses the song supposedly made by Planet Bobstar. This would mean that the theory that Bobstar is being paid by Nintendo to say these songs are fake is real, and that the songs are truly legit. Which, at the same time, could probably confirm the theory that the Wario Apparition is, indeed, a scrapped boss, or at the very least, that it has a relation to an uncut track of some sorts.

Video evidence

 * Video Evidence 1 - Breaking the Barrier
 * Video Evidence 2 - Subterranean Terror