For most people, Moscow's underworld conjures up images of high-rolling gangsters driving around in expensive cars. For Vadim Mikhailov, Moscow's underworld is a tangible place of wonder and awe, a site of endless tunnels and labyrinths, clandestine streams and strange, unexplainable phenomena. "Moscow has the largest, deepest and most complicated network of underground channels of any city in the world," Mikhailov said. In 1990, Mikhailov founded Diggers of the Underground Planet, a loose confederacy of subterranean explorers. The group first formed in Moscow, but now it has members in major cities all over the world. Mikhailov's love affair with the...