The Lost Spikes

The Lost Spikes is the nickname given to a level found in some personalized copies of Super Mario 64. This level is described as a seemingly endless rocky black and green field with several stalactites and stalagmites scattered throughout the level, being devoid of objects and enemies. This level is most commonly accessed by "entering" the skybox of Bowser in the Dark World in some way, shape or form, with some commonly reported methods being using the Wing Cap or pressing a purple ! Switch to spawn a bridge of boxes leading into the skybox to the level.

Appearance
This level takes the appearance of a rocky, black and green field covered in the stalactites and stalagmites of varying heights and sizes seen in the skybox of Bowser in the Dark World. The terrain is very rough, and it’s easy to fall into several bottomless pits. The level commonly contains no Power Stars, enemies or objects, meaning that the level could have been scrapped in a very early state or is entirely generated by the Personalization A.I.

Theorizing
It has been theorized that this level is seemingly endless due to teleporters surrounding the level teleporting the player to the other edge of the level to simulate the level being infinite, or that it was created to test the Procedural Generation anomaly, often associated with levels that appear to be endless. An alternative version of this theory is that the level is created through Procedural Generation based off of the skybox entirely by the Personalization A.I. instead of being influenced by the developers of Super Mario 64, potentially being "inspired" by the fact that Big Boo's Forest, a level believed to be genuinely unused, appears to take place in the skybox of Big Boo's Haunt.