More Urban Underground

Even though I've never tried it myself, I am fascinated with urban exploration.  This is particuarly true of forgotten underground areas, which is what I found so captivating about Downsiders by Neal Shusterman.  Many people actually explore the underground areas of New York that are used fictionally in that book.  Meet a few of them in this amazing article from NPR, then check out the website of one of the explorers.  I'm not sure I'd be up for visiting sewers, but I think abandoned subway stations and underground waterways would be cool.