This is somewhat an odd one – back in the early days of GMod 12 there was a map called GM_PortalTown, which was a modestly-sized RP map made with Portal props and prefabs, and had a sewer system made out of Portal escape sequence prefabs which the player was tricked into via a false floor. I can’t find any link to it (or even any screenshots, in fact) anywhere on the 'net. Does anyone know if it still exists anywhere?
[editline]9th December 2017[/editline]
Some more detail I remember: There was a tube system that ran throughout the under part of the map, but a certain part of it was blocked off with Companion Cubes, and when you hit it a trigger played the “Series of tubes” quote.