Our school was built in the seventies, and thus it had a large bomb shelter in the basement. A part of it was now used as a locker room, but we were always curious what else was down there.
There were multiple big rooms, separated with giant blast doors made from steel and concrete, and a couple of escape tunnels that led a dozen meters away from the school, presumably so you could escape in case the school building itself collapsed.
It was a bit tricky to get everywhere, since there were a bunch of locked doors, but we found ways to get pretty much everywhere. We didn't discover much besides old desks and chairs that were stacked in the basement, and a lot of garbage in the escape tunnels (why do people throw garbage everywhere??).
But it sure was fun!