Urbex (as in Urban Exploration) is one of those activities that fascinate me. It's sort of like Geocaching, but instead of searching for some hidden cache, you're off exploring some long forgotten piece of humanity. Oh yeah, and it's usually, but not always, illegal (what with trespassing laws and all).
While there are some well known urbex sites in DC (some I've even photographed), the general attitude of those who participate is one of secrecy. Publishing too many specifics about a location is the perfect way to get it shuttered and/or vandalized.
Want to go exploring? I found this short guide to be really helpful.
Go forth and explore - just don't forget the camera!