I'm a student at Johns Hopkins, and we have a fairly extensive steam/maintenance tunnel network. I've been down about 15 times this semester (although more focused on climbing buildings as of late), and hauled quite a few other students with. Burnt my arm pretty bad once, and got caught once but just played stupid and they let me walk. Plenty of evidence from past generations; seems to me like they were pretty popular in the 70s... who could've guessed.

You can find some of the pics here: https://www.instagram.com/jhu_liminal_spaces/?hl=en