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.