The act of going places you're not supposed to go. "Urban exploration" acts as an umbrella phrase for other terms like infiltration, draining, buildering and other actions which take place in an urban setting.
Often the act of urban exploration is illegal, as trespassing is usually necessary. However, the majority of urban explorers do not intend to vandalize property. One of the cardinal rules of urban exploration is "Take only photographs, leave only footprints".
Also known as "urban adventure" and "ue".

The act of exploring abandoned buildings, storm drains, machine rooms, and other places in cities where people do not normally go. Usually illegal, always fun. Possibly considered a terrorist act by post-democracy unelected governmental goons.

Urban Explorers are extreme sports' answer to computer hackers: a growing breed of thrill-seekers who thrive on infiltrating the planet's most dangerous man-made structures, just because they can... www.themovieafter.com

Like hiking, but in other people's buildings. Most urban adventures take place in derelict buildings, but also includes any location you can access without forcing entry.

Remember a joint and a camera to celebrate reaching the summit of your local really tall building.

Probably started by students living in halls in really big old universities who liked to explore all the tunnels and hatchs. Has even been featured on shows like Buffy and X-Files, firmly locating it in the "not cool, but fun" section of modern culture.