These huge underground networks may not see the light of day but they've been pulling visitors into their subterranean bowels for centuries.
Wieliczka Salt Mine, Poland
One of the most visited tourist attractions in Poland, the Wielicska Salt Mine first opened in the 13th century, making it one of the world's oldest. Once down in the mine, you'll find dozens of statues, as well as four chapels all made entirely out of salt by the miners.
Coober Pedy, Australia
Coober Pedy, a town situated in southern Australia, is best known as the Opal Capital of the World. But what's even more impressive about it is that the local population of 1,500 move their life underground to escape the daily heat. The town's old mines have been refurbished and now feature everything locals need during the day, like shops, kitchens, bookstores and even two churches, one of which a Serbian Orthodox Church.
Edinburgh Vaults, Scotland
The Edinburgh Vaults - or South Bridge Vaults - are a series of chambers that were used as storage space for illicit material in the 1700s, including - according to local lore - the bodies of the victims of serial killers Burke and Hare, who used them for medical experiments. Many people and paranormal investigators claim that they have encountered ghostly beings in the tunnels, most notably the ghost of a man with no face who gives off an evil presence.
Dixia Cheng, China
This huge underground shelter, known as the Underground City, was built in Beijing for fear of a Soviet nuclear attack. The shelter was made up of a sprawling network of tunnels and facilities such as movie theaters, schools, shops and warehouses. Between 2000 and 2008, it opened to foreign tourists only after years of dissuse. Since then the site has been closed for renovations.
The region of Cappadocia in Turkey alone is home to more than 200 underground cities. The city of Derinkuyu, first built in 7th century BC, is the largest of them all. With its 13 levels and depth of 85 meters, it has the capacity to shelter at least 20,000 people.