I used to do a bunch of urban exploring but haven't done so much recently. I've got a copy of Ninjalicious' book floating around the apartment that gets me all inspired to climb down in some storm drains now and again.
I mapped out a lot of steam tunnels in Chicago. Had to do something to stay warm on those winter nights.
The best was discovering a new exit, and going through it to the real world again. What was behind that door? A guard? A utility closet? An exit directly to the outdoors that I never noticed before?
Good times. They published some of my photos in my senior yearbook.