In my research, I've stumbled across a lot of interesting stuff. Perhaps the most amazing urban explorer I've encountered is amateur photographer Timothy Vogel. Although he has another job with a 50-hour week, he still finds time to explore his city. Here's a photo taken during one of his explorations.
Underneath New York, by Timothy Vogel. Click to enlarge.
In his own words: "What started as a side-project, photographing my urban exploration trips has taken on a life of its own. I think I've... become a picture-taking tourist in my own city, although not of everyday touristy things. Not quite street-photography, I describe my pictures as being city-detail oriented. The term 'urban fragment' more than often suits my pictures just fine.
My other goal is to visit places in NYC's 5 boroughs where people rarely venture, and bring back pictures seldom seen by anyone except the locals."
I doubt if many of the locals have seen what Timothy sees.
Timothy Vogel's Flickr site. Reproduced here under the creative commons: CC BY-NC 2.0
In Hartford, CN, urban explorers run the underground river in kayaks, wearing headlamps.