Beijing fills the senses when you wander its hectic streets, but there's more to it than meets the eye. Hidden from view, beneath everyone's feet, there's an underground city known as Dixia Cheng, which was built during the Cold War as a place for citizens to take shelter if nuclear weapons started flying. The sheer immensity of this labyrinth has led to comparisons with the Great Wall of China, and building it was a mammoth endeavour involving more than 300,000 citizens, many working with plain shovels rather than heavy machinery.
Access was provided through almost 100 doorways which were hidden amid normal city streets, while the hidden city had designated schools, hospital areas and residential zones, not to mention a roller skating rink and mushroom farm. Fortunately, the complex never had to be used, and now remains a bizarre subterranean ghost town.
At first glance, Corsham is a quiet, quaint market town in Wiltshire. A place of historic manors, churches and cobbled streets. But there's something more ominous hiding from view. Corsham is the site of a sprawling nuclear bunker which was intended to serve as the new government headquarters if London was destroyed by nuclear attack. This bomb-proof, radiation-proof lair was built to immense specifications so it could "comfortably" house over 4,000 people, including the Prime Minister and the Cabinet.
Now a dingy, eerie space that resembles the hideaway of a James Bond villain, the complex features office space, laundry rooms, cafeterias and a television studio for making public addresses to a terrified nation. There have even been persistent rumours the complex has its very own pub, called the Rose and Crown, although this is apparently make-believe.
Edinburgh is one of the most atmospheric cities in the world, full of ornate nooks and shadowy crannies. Some of them are very well hidden - such as the vaults beneath South Bridge. These were formed when the arches of the bridge were enclosed by other buildings, becoming storage spaces for local traders. However, flooding soon turned these dingy vaults into damp, decrepit no-go areas for the businesses above, and soon became synonymous with vagrants and criminals in the 18th Century.
Some even believe that Burke and Hare, the notorious serial killers who sold the bodies of their victims to local anatomy classes, would stalk potential victims in this hidden vaults. Unsurprisingly, these long-concealed spaces are reputed to be among the most haunted locations in the country.
Naples in Italy is famed for its historic landmarks and for being the birthplace of pizza. It's also a place of interest to urban explorers, because deep beneath the streets there's a vast space full of surreal sights. These tunnels were built in the mid-19th Century, on the orders of the (justifiably) paranoid King Ferdinand II, who wanted a quick escape route from his palace in case his subjects rose up against him.
He died before this underground route was actually completed, and it was later used to store vehicles during the dark days of World War Two. Today, the vintage cars and motorbikes can still be seen sitting in this old royal warren, shrouded in decades of dust and grime, making the tunnels a major attraction for visitors in the know.
Sacramento, the state capital of California, has a particularly unusual and fascinating underground area. It's not some disused subterranean dwelling or bunker - instead, it's the remnants of a stunning feat of the 19th Century, when parts of the city were literally raised off the ground. This was due to devastating flooding in the 1860s, which saw the buildings at risk of being swept away.
The only solution? To jack up the streets by around 10 feet, building walls and props along the original streets to create a new, higher surface level. Ground floors of buildings became basements, alleys became tunnels, and to this day you can venture beneath the pavements to see the dark, hidden maze of old Sacramento.