http://upload.wikimedia.org/wikipedia/en/thumb/f/f3/180px-Mit-signin.jpg
Roof and Tunnel Hacking is the unauthorized (generally prohibited and often outright illegal) entry into and exploration of roof and utility tunnel spaces. The term carries a strong collegiate connotation, stemming from its use at MIT, where the practice has a long history (see Wikipedia:vadding). It is a form of urban exploration.
Wikipedia:Roof_and_tunnel_hacking
At MIT "Roof and Tunnel Hacking" is a pastime where students explore the Institute where they live and work. For reasons of safety, liability, and privacy, the MIT administration isolates certain portions of the Institute from general traffic using various methods, including locks. Mastery over locks is, hence, a valuable asset to the dedicated roof and tunnel hacker
In University slang at MIT and elsewhere, Urban exploration ("roof and tunnel hacking") and pranking.