Metropolitan Area Outer Underground Discharge Channel, Tokyo, Japan – Constructed 72 feet below ground, the gigantic pressure-adjusting water tank in Tokyo looks like a concrete cathedral. 580 feet long, 256 feet wide, 59 feet high and featuring 59 pillars weighing 500 tons each, the facility took 13 years to build. Designed to take water overflow from four rivers, redirect it underground into a 4 mile tunnel before pumping it out into the Edo River, the channel is used an average of seven times a year, when heavy storms and typhoons hit the metropolitan area. When it's not preventing Tokyo from flooding, there's tours three times a day.
Pionen White Mountain Data Center, Sweden – Once a Cold War-era bunker and nuclear shelter, now the center is home to banks of computers holding secure data. What looks like a set from a James Bond film has witnessed some action in its time (albeit of the digital kind): Swedish Bahnhof, the company who runs the center, once hosted WikiLeaks servers. Buried under the mountain behind a 16 inch thick wall, it was built to withstand a hydrogen bomb.
Yerebatan Basilica Cistern, Turkey – Istanbul's Byzantine cistern, dating from the sixth century, was constructed for Justinianus I as an underground reservoir. Named after the marble columns within the chamber, the 460 feet by 230 feet room was previously a Roman basilica. The cistern fell out of favor under Ottoman rule (who preferred running water), and it was only 'rediscovered' in the sixteenth century by Dutch traveler P. Gyllius while roaming the Hagia Sofia nearby. Households above the cistern had been drawing water from 'wells' cut into their ground floors -- some were even known to fish. Gyllius, a veritable Indiana Jones, navigated the waters on a rowing boat with a lamp. Today the cistern is uplit and easily accessible for visitors.
Bounce Below, Wales – A 176-year-old disused slate mine near Blaenau Ffestiniog, North Wales became a subterranean playground in 2015 when Bounce Below opened. The $920,000 investment installed a giant, multi-tiered trampoline network suspended in a cavern the size of a cathedral. Despite the soft landing, hard hats are still required.
Down Street station, England – London Underground is littered with disused stations throughout its network -- over 40, at last count. Down Street station, in upmarket Mayfair, opened in 1907 but closed in 1932, authorities citing its lack of popularity. Before Churchill's Whitehall bunker was built the cabinet would meet in the disused station in the years leading up to World War II, when it was known as "The Barn". Since then it has remained abandoned, although in 2015, Transport for London reached out to developers, who suggested the space could be used for everything from bars and restaurants to gallery or theater space.
Cabinet War Rooms, England – Down Street station was abandoned for the Cabinet War Rooms in Whitehall a week before World War II commenced. The basement hosted 115 cabinet meetings -- mostly during the Blitz -- and was in use around the clock until August 16, 1945, when lights in the Map Room were switched off for the first time in six years. Preserved entirely unchanged since the conflict ended, the Imperial War Museum has opened up the bunker to the public, where visitors can retrace the steps of Churchill's victory.
Greenbier Resort, US – In White Sulphur Springs, West Virginia is Greenbrier Resort, a resort like no other. Mainly because under one wing is a 112,00 square-foot shelter originally designed as a evacuation point for US Congress staff in the event of a nuclear attack on home soil. Completed in 1961 and operational until 1995, behind its 25-ton blast door is a warren of tunnels and rooms containing everything from bedrooms to a dentist surgery. In 2006, after a two year renovation, the former Cold War relic opened to curious patrons.