A Nazi hideoutAn ordinary door in Berlin's Gesundbrunnen train station leads to an abandoned air raid shelter that reveals Berlin's dark underbelly: an array of secret bunkers, escape tunnels—even an aircraft factory—built by the Nazi regime during WWII and expanded during the Cold War. Berlin Underworlds' Association, 011-49/30-4991-0517, berliner-unterwelten.de, 90-minute tour $13.
Paranormal activityInch through cramped corridors dug below Edinburgh's South Bridge in the 1700s that were originally used as storage spaces by merchants. The corridors became a crowded slum and eventually a rumored 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, 011-44/131-225-5445, mercattours.com, 75-minute tour $12.
Water worldThe Basilica Cistern, which once supplied water to Topkapi Palace in Istanbul, is a cavernous space outfitted with imposing domed ceilings and towering Corinthian columns—two are mounted on giant Medusa head bases, one of which is inexplicably facing upside down. Five additional subterranean cisterns (some ruined, some restored) are explored on this tour. Les Arts Turcs Tours, 011-90-212-527-6859, bazaarturkey.com, 4-5-hour tour $37.
Herod was hereAncient tunnels beneath the heart of Jerusalem's Old City expose layers of the Western Wall, including a gigantic 570-ton stone—one of the heaviest objects ever lifted without machinery. Visitors can see the remnants of the temple built by King Herod about 2,050 years ago, and walk along a road that ran next to the temple in Herod's time. Book tours at least two months in advance. The Western Wall Heritage Foundation, 011-972/2-627-1333, english.thekotel.org, 75-minute tour $7.
Even the graffiti's oldA gate hidden in an alleyway behind Naples's Piazza San Gaetano reveals a network of nearly 250 miles of caves and tunnels constructed two millennia ago. Still lying among the ruins are a Greco-Roman theater submerged underneath houses and ancient aqueducts now lined with WWII-era graffiti. Napoli Sotterranea, napolisotterranea.org, 011-39/081-296-944, two-hour tour $14.
Unscheduled stopWhen 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 downtown Manhattan, seen only on this several-times-yearly tour, which includes a look at a 100-year-old train car. New York Transit Museum, 718/694-1600, mta.info/museum, one-hour tour $20 (annual museum membership required; $40 per person, $55 for a family of four).
Sewer cityFans of Les Misérables are familiar with tales of vagrants hiding in the City of Light's pitch-black sewer system. Originally built in the 1200s, these Paris sewers were greatly expanded by influential civic planner Baron Georges-Eugène Haussmann in the 1850s and today can be accessed via a museum between the Quai d'Orsay and the Seine River. Musée des Égouts, 011-33/1-53-68-27-81, parisinfo.com, one-hour tour $5.50.
Illegal marketExplore a rumored entrance to the legendary Shanghai Tunnels beneath Portland's Chinatown, where transient laborers were allegedly kidnapped and sold into slavery in the late 19th and early 20th centuries. (Historians now question whether such events actually occurred.) Look for the 1880 coal chute and the remains of a coal stoker furnace. Portland Walking Tours, 503/774/4522, portlandwalkingtours.com, two-and-a-half-hour tour $19.
Dan Brown's inspirationFrom ancient Christian burial grounds to underground dungeons, enough secrets lie below Rome to form the basis of another Angels and Demons. The tour highlight is the multilayered 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, 011-39/041-970-499, tours-italy.com, March–October, three hour tour $110.
Burnt outAfter 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 because of fears of bubonic plague, but today visitors can walk along sidewalks, streets, and storefronts, some still intact and some restored. Bill Speidel's Underground Tour, 206/682-4646, undergroundtour.com, 90-minute tour $15.
Plot foiledThe 3rd Infiltration Tunnel, a narrow, mile-long shaft below the Korean border, was dug in secret by North Korea, apparently designed for a surprise attack on the South. Discovered by South Korea in 1978, the tunnel is just 32 miles from Seoul. Grace Travel, 011-82/2-332-8946, triptokorea.com, part of the six-hour DMZ Transit Tour $38.
Through the agesBeneath the 2,500-year-old city of Vienna 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, 90-minute tour $24.
