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urban speleology
Also known as urban exploration, urbex, or UE; it is the examination of the normally unseen or off-limits parts of human civilization.

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.

In the U.S.A., it may also be referred to as "draining" (when exploring drains) "urban spelunking", and "urban caving."