If you're not careful and you noclip out of reality in the wrong areas, you'll end up in the Backrooms — roughly 600 million square miles of randomly segmented empty rooms, all built from the same yellow wallpaper, all floored with the same moist monocolor carpet, all lit by the same buzzing fluorescent lights. There is nothing to do in the Backrooms, and no way out.
The original 4chan post read like a survival guide:
"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."
The image attached to the original post — a grainy, slightly fisheye photograph of an endless corridor of yellow rooms — is why it spread. The photograph didn't look constructed. It looked like a place that existed somewhere, a place no one was meant to photograph. The specific quality of the lighting, the specific shade of yellow, activated something uncomfortable in most viewers. It looked like somewhere you'd been. Somewhere you almost remembered.
The community that formed around the Backrooms expanded the mythology considerably. Levels were catalogued — Level 0 being the yellow rooms, followed by progressively stranger and more hostile environments. Entities were described: Smilers, Hounds, Partygoers, the Skin-Stealers. Survival rules were compiled: don't make loud noises, don't enter water, don't follow sounds of music, don't look at Smilers, find a camp and maintain it.
But the original post requires no mythology. It requires only the image and the description and the word "noclip" — the exploit that lets you pass through geometry in video games, to fall into the space behind the walls of the world.
The concept that there is a space behind the walls of reality. That you can fall into it by accident. That it is enormous and empty and lit by lights that were never meant to illuminate anything and never turn off.
That there is something in there that has heard you.
That is enough.