We found a bizarre hidden interrogation room during college.

The basement of a building on campus was only accessible by a service elevator or emergency stairs. It was a typical dark, industrial basement with exposed wiring and ducts across the ceiling, old dusty pipes occasionally going down the walls, only a few incandescent lights here and there, making it pretty dark.

Down a hall, there were a couple locked rooms; one we used to store lab equipment. Nothing out of the ordinary, except the end of the hall was pitch black. We came back later with flashlights, because it was creepy and why not. Turned out the hall kept going around a corner, and the ceiling sloped downward to a short, maybe 3.5' door at the actual end of the hall. It was like that scene in Charlie and the Chocolate factory where it sorta looks normal, except the hall gets tiny and you have to duck to get through door at the end, only in a 50+ year old dusty, industrial basement.

The door at the end was locked with a deadbolt, and the handle was missing so you could look through. Inside, it looked to be a room about 10' by 10' with full height ceiling despite the weirdly short door. There was a pull-chain light hanging from the ceiling that was left on. It looked like some metal sheets and bars were leaning against a wall, and there was a single folding chair in the middle of the room facing the door. It was creepy to say the least.

We checked back every time we had to go down there, and the light was never on again, so someone must've been in there at some point. Shining a light through the door-handle-hole, it looked like the chair had been moved somewhere else too, but nothing else happened.

So not exactly secret I guess since it was probably just another storage room at the end of a hall... just a pitch black, funhouse/horror-movie hall with an interrogation room at the end.