Urban spelunking

Urban exploration (spelunking) is the examination of the normally unseen or off-limits parts of urban areas or industrial facilities. Urban exploration is also commonly referred to as infiltration, although some people consider infiltration to be more closely associated with the exploration of active or inhabited sites. It may also be referred to as "draining" (when exploring drains) "urban spelunking", and "urban caving", "vadding", "trolling", "building hacking", "reality hacking" or "roof and tunnel hacking". This form of hacking can also take place in 'active buildings'.

Check out:
http://weburbanist.com/2007/08/05/urban-exploration-beginners-guide-to-adventures-in-building-infiltration/
http://www.infiltration.org/abandoned.html