Unused Level 35

Unused Level 35, filename 35POISON.bin, is an unused level in Super Mario 64. It has no files associated with it in the main game, meaning it is unable to be loaded. It can however be seen in the debug level select. The July 25th 2020 leak shed some light on this level, having some of its assets. However, the level is merely a large checkerboard plane with nothing else to it.

Theorising
It's possible that, like Unused Level 2 or Unused Level 32, that this level was intended for testing purposes. Judging by the filename, 35POISON.bin, it could've been intended to test the poisonous gas effect used in Hazy Maze Cave that causes Mario to choke and lose health slowly. But the level itself doesn't reflect this, only featuring a flat plane for Mario to run around across.

Alternatively, this level could've been home to an unused level concept featuring poisonous gas. It's possible the idea for such a level was cut very early on the game's development, so it never was worked on.