Vaguely inspired by the looping rooms of Submachine:
Essentially from the surface, it's a hatch leading down into a medium-sized (10m by 10m?) room. Typical war bunker: concrete/steel walls, industrial lighting, etc. Each wall has a door in the center, though you can't see through the door. When you walk through the door, you enter another room, identical to the first, and occupying the same physical location as the first. As you keep going, it's just an infinite grid of these identical war-bunker rooms. Only the first one has an exit to the surface though, so if you get lost you're probably dead.
Any changes made to a room will persist upon exiting and re-entering, except for severe damage to the walls and ceiling/floor. Also, it's impossible to enter another room except through the designated door (ie you can't break through a wall into the next room).
So the story would be that the Foundation finds this place and sends some teams to explore. They find quite a few corpses, as well as labs and research notes. It turns out that this place was not only a war bunker, but also a secret research facility. And the real twist: One set of notes implies that the rooms were created with SCP-184. They didn't create 184 itself, but they found it and found a way to perfect its effects.