

Any Palahniuk fans may have heard of this in 'Fugitives and Refugees' where Portland Cacophony did guided sewer tours. Well Urban Exploration is in this spirit.
Firstly, Urban Exploration has no set boundaries, it covers a huge spectrum of places and has no limits. Exploring ranges from disused asylums/hospitals, bunkers, drains, industrial sites to cranes to infiltration of live sites.
Urban explorers usually want to visit places that most others do not want to go, we do this in most cases to document and photograph our heritage and to explore the normally unseen or off-limits parts of human civilization.
Saying this urban exploration does also cover places that are open to the public, there's nothing wrong with exploring say a restored fort or a museum, its whatever you want it to be.
Always know your limits, don't try anything too intense if you don't feel happy with it, and never feel pressurised into doing something you don't want to do.
There are no set rules in Urban exploration, most things go, however you should never break in anywhere and never steal anything.
http://www.sub-urban.com/
http://www.guerillaphotography.fotopic.net/
http://www.28dayslater.co.uk/forums/index.php