The Secret Coffin

The Secret Coffin, also known as Big Boo's Coffin, is a supposed level in some personalized copies of Super Mario 64, accessed through the coffin found at the entrance of Big Boo's Haunt. It has been described as a level very similar to the Secret Aquarium, appearing to imply it was an earlier version of the level or that the level was created through Dynamic Level Arrangement, reusing the Secret Aquarium model.

Description
The Secret Coffin is described as an altered version of the Secret Aquarium with no water, occasionally with a raised ceiling, likely to reflect the coffin the level takes place inside of being taller than the regular box the Secret Aquarium inside of. The Secret Coffin usually contains a mass amount of Boos, potentially referencing the large amount of boos in one of the Ghost Houses found in Super Mario World, and other enemies in Big Boo's Haunt such as Scuttlebugs and Mr. Is, and occasionally even Dry Bones.

The level has had several stars reported, with the most common being an 8 red coin star, similarly to the Secret Aquarium, a 5 secrets star, a star for defeating a Big Mr. I, and a star for defeating a Big Boo, occasionally being spawned after a certain amount of the Boos are defeated, similarly to the first mission of Big Boo's Haunt.

Report 1
This video was uploaded recently and was the first documentation of the Secret Coffin. It begins at the Castle Grounds right outside of Princess Peach's Castle before entering and running to the Castle Courtyard, passing by a Toad with blue spots and a box in the Castle Courtyard. Upon entering Big Boo's Haunt, all objects appeared to disappear and the stairs were active despite the first mission being selected. The player then entered the Secret Coffin. Inside, dozens of Boos resided and flew around, strangely with some being tilted. The player defeats all of the Boos, collects 8 red coins, and collects the level's Power Star as the video ends.