Unused Level 38

Unused Level 38, filename 38POOL.bin, is one of the unused levels in Super Mario 64. Much like the other six unused levels, this one can be found in the level select of the base game, although without any name and without an actual level to play on. In the July 25th 2020 leak, files associated with the level were found such as the level's model. The level features sand terrain that would later be used in Shifting Sand Land, along with the object that spawns a mirror Mario to be used in the mirror room.

Theorising
The internal name POOL may indicate that this level was planned to be replaced with a water level, like an actual pool. It may also be that the internal name POOL is a reference to how the test level looks like a billiards table as used in games of pool (as it is very green) This also raises the possibility that an actual billiards-themed level was meant to replace this level's slot later in development.