12 Truly Underground Tours

Berlin: An unsuspecting door in the Gesundbrunnen train station leads into an abandoned air raid shelter that reveals Berlin's dark underbelly: an array of secret bunkers, escape tunnels—even an underground aircraft factory—built by the Nazi regime during WWII and expanded during the Cold War. Berlin Underworlds Association, berliner-unterwelten.de, 011-49/30-49-91-05-17, $13, 90 minutes.

Edinburgh: Inch through cramped corridors dug below South Bridge in the 1700s originally used as storage spaces for merchants. They became a crowded slum that housed the city's poorest residents and eventually a dumping ground for murder victims, which may explain why more than a few visitors have reported paranormal sightings in these creepy, claustrophobic passageways. Mercat Tours, mercattours.com, 011-44/131-225-5443, $12, 75 minutes

Istanbul: The Basilica Cistern, which once supplied water to the Topkapi Palace, is a cavernous space outfitted with imposing domed ceilings and towering Corinthian columns mounted on giant Medusa head statues. It's one of six subterranean cisterns (some ruined, some restored) explored on this tour. Admission to the Basilica Cistern is $6.80; Les Arts Turcs Tours, bazaarturkey.com, 011-90-212-527-6859, $37, 4 hours.

Jerusalem: An ancient tunnel beneath the heart of the Old City exposes the foundation of the Western Wall, including a gigantic 500-ton stone—one of the heaviest objects ever lifted without machinery. Visitors can see the remnants of the 3,000-year-old temple built by King Herod, and walk along a road that ran next to the temple in Herod's time. Tours should be booked at least two months in advance. The Western Wall Heritage Foundation, english.thekotel.org, 011-972/2-627-1333, $7, 75 minutes.

Naples: A gate hidden in an alleyway behind Piazza San Gaetano reveals a 60-mile network of caves and tunnels constructed two millennia ago. Still lying among the ruins are a Greco-Roman theater submerged underneath a typical home and ancient aqueducts now lined with WWII-era graffiti. Napoli Sotterranea, napolisotterranea.org, 011-39/081-29-69-44, $13, two hours.

New York City: When it opened in 1904, the original City Hall subway station wowed commuters with its intricate brass chandeliers, leaded skylights, and arched Guastavino ceiling tiles. Today it lies forgotten beneath lower Manhattan, seen only on these several-times-yearly tours, which include a look at a 100-year-old train car. New York Transit Museum, mta.info/museum, 718/694.1600, $20, one hour; an annual membership to the museum is required; $40/person or $55 for a family of 4.

Paris: Fans of Les Miserables are familiar with tales of vagrants hiding in the City of Light's pitch-black sewer system. Originally built in the 1200s, the sewers were greatly expanded by influential civic planner Baron Haussman in the 1850s, and today can be accessed via a museum between the Quai d'Orsay and the Seine. Musée des Égouts, 011-33/1-53-68-27-81, parisinfo.com, $5.60, one hour.

Portland: Explore the seamy basements and rumored entrances to the legendary "Shanghai tunnels" beneath Chinatown, where transient laborers were allegedly kidnapped and sold into slavery in the late 19th and early 20th century. (Historians now question whether such events actually occurred.) Portland Walking Tours, 503/774/4522, portlandwalkingtours.com, $19 dollars, two hours.

Rome: From ancient Christian burial grounds to underground dungeons, enough subterranean secrets lie beneath the Eternal City to form the basis of another Angels and Demons. The tour highlight is the multi-layered Basilica di San Clemente: A 12th-Century basilica built on top of a 4th-century church, which was built next to a 3rd-century temple—today, all three areas can be explored. Avventure Bellissime Tours, tours-italy.com, 011-39/041-970499, $109, three hours, March–October.

Seattle: After the Great Fire of 1889 destroyed 25 city blocks, Seattle rebuilt one story higher because of flood concerns, pushing the charred remains below ground level. In 1907, the underground city was condemned for fear of pneumonic plague, but today visitors can walk through many of the still-intact sidewalks, streets and storefronts. Bill Speidel's Underground Tour, undergroundtour.com, 206/682-4646, $15, 90 minutes.

Seoul: The Third Infiltration Tunnel, a narrow, mile-long shaft below the Korean border, was dug in secret by North Korea, apparently designed for a surprise military attack on the South. Discovered by South Korea in 1978, the tunnel is just 32 miles from Seoul. Grace Travel, triptokorea.com, 011-82/2-332-8946, part of the six-hour DMZ tour, $38.

Vienna: Beneath this 2,500-year-old city is a maze of medieval cellars, preserved Baroque crypts, excavated Roman ruins and underground passageways. While they were built over several centuries, many of these sites are now connected by tunnels that served as air-raid shelters during WWII. Vienna Walks & Talks, 011-43/1-774-8901, viennawalks.com, $24, 90 minutes.