At Carleton University in Ottawa, there are tunnels connecting every building, and accessable to everyone. Very nice in the winter time, espessially living on campus. There are also a network of side tunnels from these that hold steam pipes, water, power, data, etc. Usually they're all behind doors locked with deadbolts, but sometimes they're left open or otherwise accessable. We spent a few nights exploring some of them, but unfortunately never took pictures.
There are a bunch of electrically-powered "tunnel carts" that maintenance workers use to get around campus. Ocasionally they have been found unlocked, seemingly parked and forgotten about at night. Of course, sometimes they belong to security, and well.. that's a story that I won't get into now (I was not directly involved with this).