There are all these awesome steam tunnels under NC State University in Raleigh. We used to go over there and run around in them when I was in high school. There was just this one manhole cover in front of this one building, you lifted it up and down you went.
What stopped us was when we saw this plastic sheet that had "Danger! Asbestos" printed on it stretched across the hallway - and we were reading it from the wrong side.