In most urban areas, exploring the underground infrastructure on your own could land you in some serious trouble with the law. But there are plenty of safe and legal ways to experience la vie sous-terraine.

STATESIDE TOURS: Trek through tunnels to a former brothel and opium den with Oregon's Pendleton Underground Tours ($10; www.pendletonundergroundtours.org), three hours east of Portland. In Seattle, the 19th-century streets of Pioneer Square lie beneath the present-day city. Explore them with Bill Speidel's Underground Tour ($11; www.undergroundtour.com).

PARIS MUSEUM: Delve into 2,985 feet (910 meters) of tunnels at Les Catacombes museum, the only lawful way to visit the city's sprawling labyrinth. It's not for the squeamish: The museum's rock-hewed walls are lined with the remains of more than six million Parisians ($6; www.paris.org/musees/catacombes/info.html). 

ARMCHAIR UNDERGROUND: Caroline Archer's photo-packed Paris Underground (Mark Batty, $45) documents 500 years of catacomb art. Author Julia Solis bites
into the Big Apple's core in New York Underground (Routledge, $36). 

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