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Kashan, Iran - Located north of Kashan is a 1,500-year-old masterpiece of Sassanian architecture and engineering, an underground city called Ouyi where residents would hide from hostile intruders. Some of the tunnels and chambers are open to the public.

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Derinkuyu, Turkey -

Completed during the Byzantine era, the extraordinary underground city at Derinkuyu could accommodate up to 20,000 souls and proved a formidable defense against attack. Much of the ancient complex is accessible to visitors.

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New York City, USA -

Blockhouse No 1 in New York's Central Park dates back to the early 19th century and the War of 1812. Essentially a two-story bunker, it is the oldest building in the park.

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Gibraltar, UK -

A remarkable 34 miles (55 km) of tunneling has been burrowed out of the Rock of Gibraltar since the late 18th century. Over the years, the Rock housed hospitals, barracks, and ammunition stores. Most are now decommissioned, with some open to the public.

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Alderney, UK -

One of the more eye-catching bunker facilities built by occupying German forces on the Channel Islands is this fortification, known locally as "The Odeon."

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Pointe du Hoc, France -

The promontory in Normandy was a strategic location during WWII, the highest point between Utah Beach to the west and Omaha Beach to the east. The coast is littered with concrete bunkers and casemates.

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Colmenar del Arroyo, Spain -

This characteristic bunker installation was built by Nationalist forces during the Spanish Civil War.

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Cabinet War Rooms, London -

One of the UK's most popular visitor attractions, Churchill's secret bunker, sited in Westminster, has been preserved in its original state.

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Skanderborg Forest, Denmark -

The remains of the WWII German Luftwaffe headquarters in Denmark include four bunkers, two of which have been adapted as museum exhibits.

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Cap d'Antifer, France -

The Normandy coast is strewn with bunkers, remnants of the Nazis' infamous Atlantic Wall defensive fortifications.

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Landrethun-le-Nord, France -

Part of the imposing Forteresse de Mimoyecques, the sprawling bunker complex set underneath this quiet corner of rural France was built to house the Nazis' deadly V3 rocket. Fortunately, this terrible new weapon failed to launch.

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Cape May Point, USA -

Its incongruous setting on a New Jersey beach makes this abandoned WWII bunker a local landmark.

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Jersey, Channel Islands - The only British territory to be occupied by the German Army during WWII, the island group includes Jersey, the location of the Jersey War Tunnels. The tunnels were designed to withstand Allied air raids and bombardments.

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Liepāja, Latvia -

Known as the Northern Forts, this subterranean labyrinth of tunnels and bunkers forms part of a fortress that dates back to Tzarist Russia.

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Moscow, Russia -

Bunker 42, a 1950s-era secret Soviet bunker sunk beneath the Russian capital also serves as the Cold War Museum.

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Līgatne, Latvia -

Built during the Cold War for the Communist elite, rural Līgatne's underground nuclear shelter remained secret for 30 years.

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Konjic, Bosnia and Herzegovina -

In 1953, Yugoslav communist revolutionary and political leader Josep Broz Tito (1892–1980) ordered the construction of a nuclear bunker inside a mountain. Today it's a funky modern art exhibition space. 

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Elbasan, Albania -

Likewise, during his iron-fisted rule, Communist dictator Enver Hoxha (1908–1985) ordered the construction of thousands of bunkers, which today lie in abandon ruin across the country. 

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York, England -

Operational from the early 1960s right up until the mid-'90s, the York Cold War Bunker was designed as a nerve center to monitor fallout in the event of a nuclear strike. It's now a novel tourist attraction.

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Moscow, Russia -

Stalin's Bunker dates back to WWII. The Russian dictator would have planned some of the bloodiest battles in history from this simple but goosebumps-inducing shelter.

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Budapest, Hungary -

Set beneath Buda Castle Hill, the city's Hospital in the Rock served as an infirmary during WWII but was adapted as a nuclear bunker during the Cold War years. Its engaging museum explains more.

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Cheshire, England -

A Cold War secret that remained hidden for 50 years, Hack Green Secret Nuclear Bunker welcomes visitors curious about this former government-owned subterranean command center.

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Fife, Scotland -

Scotland's Secret Bunker lies 30 meters below a nondescript farmhouse and is the size of two soccer fields! It's now one of the region's most compelling visitor attractions.

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Kelvedon Hatch, England -

For years the leafy borough of Brentwood concealed one of the biggest and deepest Cold War bunkers in the country. A tour affords a fascinating behind-the-scenes glimpse of this former hush-hush facility.

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Ho Chi Minh City, Vietnam - Veterans from both sides of the conflict mingle with curious tourists to delve inside the Củ Chi tunnels, an immense network of corridors and bunkers that are now part of the city's Vietnam War memorial park.

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Quảng Tri, Vietnam -

Similarly, the Vịnh Mốc tunnels can be visited as part of an organized tour.

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Berlin, Germany -

History's most infamous bunker complex is that which Hitler retreated into during the last weeks of WWII. Pictured is a US serviceman in what is believed to be the Führer's study, the room where the defeated German dictator and his wife Eva Braun are thought to have ended their lives. 

If you enjoyed this gallery, check out: The World's Top 30 Military Powers Today

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