The Vanish Sewers were a subterranean tunnel system that displayed signs of sentience from an Undefined Multiverse.
They were fairly normal sewers, at least by Human standards, until the arrival of two human repairmen named Phil and Hank. Things went smoothly, until Phil noticed the arrival of the Molemen. Shortly after their arrival, the sewers began to arrange themselves of their own volition, trapping those within with dead ends and looping corridors, allowing them to be picked off by the Molemen.
The sewers were also distorted by the tunnels dug by the Molemen, though when it rearranged itself, it often covered up the tunnels with walls. Other humans on the outside took to trapping people within, as a method of execution.
Markiplier had extreme difficulty escaping the sewers.