The tunnels at
Caltech can easily be found by looking for a "Danger:
Asbestos Hazard" sign on otherwise unmarked doors in the subbasements of most buildings.
The steam tunnel system connects nearly all the buildings on campus and is often used by frosh retreiving their chemistry homework from Gates Lab at 4 am.
Getting Caught
Caltech security is rather leinient--since most undergraduates have "south" master keys and the honor code here actually works, just make sure to have your student ID with you and they'll let you go. However, the chances of actually meeting security are slim to begin with as long as you avoid the round tunnels, which contain motion detectors.
Sightseeing
Due to the lax security over the decades, the tunnels have accumulated a lot of artwork. There are several large murals throughout the tunnels, as well as random graffiti and bad poetry nearly every foot of the walls and steam pipes. DEI, FEIF, GDBG, and Beward of the Leopard are popular. There's also a large "Caltech sucks" from some Harvey Mudd students who got themselves lost in the tunnels for several hours.
Hazards
Watch out for the radioactive rocks the geology department stores down there. There are also, of course, hot steam pipes and asbestos. It would be interesting to do a study on rates of lung cancer in Caltech alums.