When you next walk through the streets of a major European city, spare a thought for all that might be lurking just under your feet. Beyond the obvious public transportation networks, electricity grids and water canals, you may well be strolling over bones of plague victims and buried artifacts. For a true change of pace, why not visit that underworld? Here are some cities that offer tours of what lies beneath:

Berlin — The city’s public transportation network now offers a night-time tunnel ride covering a distance of some 25 kilometers and acquainting passengers with the history of the Berlin metro. Helmets are mandatory, and you must be over 18 years of age. The tour costs 40 euros and lasts about two hours.

Budapest — Beneath Buda Castle can be found an extensive labyrinth of caves and cellars. After 6 p.m., you can tour the grounds by the light of oil lamps.

Edinburgh — Mercat tours is just one company that organizes tours of the city’s underground, including 18th-century vaults that once served as housing beneath the South Bridge. They also offer a “haunted underground experience.”

Paris — The catacombs of Paris, a former quarry in which bones from the city’s cemeteries were deposited from the late 1700s, can be visited daily except on Mondays; alternately, you can tour the Musée des Égouts, or Sewer Museum, and watch — and smell — torrents of waste water gush past. The official site is in French only, but My Trudge through the sewers of Paris on the Epinions Web site gives a colorful description of what you might be in for.

Naples — Napoli Underground is a wealth of information about the city’s many subterranean sites, including an extensive underground cemetery. Venturing outside city limits, sites you can see include an active volcano crater, grottoes and cisterns.

Rome — Seems every time officials try to expand the city’s metro system, construction projects are delayed as they bump into yet more archaeological remains. There’s a whole lot going on below decks in Rome. About.com’s Explore subterranean wonders beneath Rome is a good place to begin the descent.

Vienna — Visitors can take part in "The Third Man" tour. Descent is through a manhole, followed by a walk through the historic vaults of Vienna’s sewer.

Outside big cities —Other underground worlds of a different nature but worth exploring, (and, incidentally, included on the UNESCO list of world heritage sites) include Cappadocia, Turkey; Škocjan caves of Slovenia; Wieliczka Salt Mine outside Kraków, Poland; the Sassi of Matera, Italy; and the Monastery of the caves in Kiev, Ukraine.

If you’re really curious about the worlds beneath our feet, you’ll be fascinated to learn about the upcoming International Conference on Underground Spaces - Design, Engineering and Environmental Aspects to be held in the U.K. in 2008.