The Basement is the lowest level of the Backrooms, presumably Level -9,223,372,036,854,775,807. It is also presumably the largest, underpinning all other Backrooms levels. (For more info on how one infinity can be larger than another, see here.) If one digs or crawls or swims deep enough on any Backrooms level, one can no-clip into the Basement. Finding a place to no-clip out, however, is almost impossible.

The Basement resembles some kind of underground structure, that is both being demolished or has been ruined by a disaster, and under construction or being repaired. Floors, walls, and ceilings are often uneven and incomplete or cracked, there are open pits and ceilings, stairways sometimes lead to a ceiling or wall, and there are many spaces too small to get through. Some of the floors and stairs may break if stood upon, and there are mounds and slopes of dirt that aren't always stable. Dead ends abound, but most rooms have multiple exits and passageways meander endlessly. Lighting exists, but it can be rare and doesn't always function properly or at all. The general structure does not resemble any comprehensible purpose for a building, including the sheltering of human life.

The only "entities" that exist in the Basement are occasional rats and mice, cockroaches, and often poisonous spiders. Unfortunately, the former of these are the only source of food. Most of the level is dry and dusty, but there are puddles in some places and the occasional room is flooded. The filthy water in these cases are the main source of hydration. Rarely one may find a leak in a pipe or a working faucet with clean water. Some of the wires and cables provide electricity, but they are just as likely to be dead or dangerously worn.

The level smells of dirt and sawdust, and the temperature is a consistent 67° F. There are no beds or tables, but one may find the occasional piece of construction equipment or a folding chair.

If you end up here, you are probably going to die alone, in the dark and silence, and no one will ever find your remains.