Beneath all cities, lies an alternate dimension, not necessarily the "dark, underground, loathsome vice" of Dostoevsky (though there's plenty of that, too), but a real warren of subterranean spaces that is mostly hidden or off-limits to surface dwellers. Many communities were intentionally founded above natural caves, which proved useful for food storage, refrigeration, defensive purposes and escaping extreme weather. Other towns were carved into solid rock or set atop tunnels for protection from invasion.
These underground spaces tell a ramshackle story of human civilization and urban development from poorly conceived shopping malls to wine cellars, escape routes, storage facilities for human bones, and conduits to whisk away human waste. These days, thanks to sophisticated mapping programs and social networking, nearly every city has a legion of underground spelunkers looking to make the next discovery of a forgotten entrance to the urban underworld. But you don't have to trespass or risk your life to explore the world beneath your feet -- plenty of towns are taking a cue from the 19th-century Paris of Victor Hugo, offering opportunities to embark on underground tourism.