Level 0
"The Lobby"
"Level 0 is an expansive non-Euclidean space, resembling the back rooms of a retail outlet. All rooms in Level 0 share the same superficial features, such as worn mono-yellow wallpaper, old moist carpet, scattered electrical outlets, and inconsistently-placed fluorescent lighting. Aside from these common features, no two rooms within the level are identical. The fluorescent lighting present in Level 0 hums at a constant frequency. This buzzing is notably louder and more obtrusive than ordinary fluorescent lights, and examination of the fixtures to determine the source has been inconclusive thus far. Secondly, the substance lightly saturating the carpet cannot be consistently identified; it is not water, nor is it safe to consume." ¹
Origin
This level, (being the only one), had its origins on 4chan, the original caption read "If you're not careful and you noclip out of reality in the wrong areas, you'll end up in the Backrooms, where it's nothing but the stink of old moist carpet, the madness of mono-yellow, the endless background noise of fluorescent lights at maximum hum-buzz, and approximately six hundred million square miles of randomly segmented empty rooms to be trapped in God save you if you hear something wandering around nearby, because it sure as hell has heard you." It was a part of the /x/ board originally and it became incredibly popular online which then lead to the popular series "backrooms" by Kane Pixels.
Level 0, and by extension, the Backrooms, can be accessed by accidentally no-clipping out-of-bounds in normal reality. In addition, many levels can lead one to Level 0 ¹.
In-Game
In level 0 you can gain x achievements
In order to escape Level 0 you need to explore the vast area of Level 0. Be careful of The Howler(s), they usually roam the exit and the entrance area. The exit usually looks like a ladder leading straight to Level 1.