The Backrooms is a Creepypasta that originally started with one post, then became a hit that branched out into multiple different continuities on different websites and wikis:
Originally, this image of an empty office area got posted in response to a thread about images that looked inherently unsettling. Someone replied to it with their own idea, dubbing it "The Backrooms", an essentially endless maze of the same repeating office rooms and hallways that could only be entered if one "noclips" through reality itself, with implications that there may either be monsters living somewhere within the maze or that those trapped there will be driven mad to the point where they begin to hallucinate things that aren't there.
Eventually, the eerie concept spiraled out into various stories and apparently at least two separate websites/wikis that explored the idea of the Backrooms having various other levels, all of which were some kind of liminal spaces with eerily uncanny realism, some of which even seemingly take place outdoors along with the introduction of various "Entities", monsters of many different varieties that lurked that Backrooms, most of which are extremely dangerous, though a few are actually friendly and helpful. Since there's a few different websites dedicated to the Backrooms, though, the levels and entities you can read about are probably inconsistent.
To top it all off, ever since a short film uploaded by Youtube user Kane Pixels about the Backrooms went viral, the Backrooms has became even more of a hit. Oh, and did I mention that Kane Pixels' version of the Backrooms follows yet
another separate continuity?
Yeah.....Like I said, it's complicated lol. It's definitely a refreshing take on horror, but based on what I've read about it so far, this still isn't even the tip of the iceberg.