816 Nuclear Military Plant (pictured) was designed to manufacture plutonium in the 1960s, but despite being stopped from doing so in 1984, it hasn't been open to the foreign public until this year
More than one million square feet.
It could be straight out of a Bond movie.
This vast underground cavern, formally a top-secret Chinese nuclear base, has finally opened for tourists to have a poke around.
Hailed as the largest man-made cave in the world, the 816 Nuclear Military Plant, located in the mountains of Fuling district in China's Chongqing municipality, was designed to manufacture plutonium in the 1960s - a 17-year project involving more than 60,000 soldiers.
While it was stopped from going into operation in 1984, the base wasn't declassified until April 2002, and it hasn't been open to foreign visitors until now.
Yang Yan, an administrator at the site, told China Daily: 'So far, no foreigners have visited the plant. A tour takes three hours, and visitors must follow the guide, otherwise they will get lost in this maze-like cave.'
The enormous space spans more than 12 miles, with 18 main caves and more than 130 roads and tunnels, which vehicles are free to pass through.
It was designed to tolerate thousands of tons of TNT explosives, and at least 100 workers were reported to have died during construction.
These days, it's been renovated to feature colourful modern lighting, as well as educational displays. And visitors need not fear for their safety.
'This base has never been put into operation or stored any nuclear material,' Mr Yang said. 'There is no need to worry about radiation. It is safe to enter.'
The historic base is so enormous at more than one million square feet, that tours take three hours and must be led by a guide. dailymail
It was never put into operation, despite taking 17 years to be built, nor has it ever stored nuclear material, but it has since been renovated to feature light displays
The enormous space spans more than 12 miles, with 18 main caves and more than 130 roads and tunnels to wander around in
The site boasts educational displays, and hopes to attract its first visitors from overseas - no foreigners have been thus far. dailymail
It was designed to tolerate thousands of tons of TNT explosives, and at least 100 workers were reported to have died during construction
Zhang Xiaodong, the former assistant manager of the Chinese military plant, takes viewers through its engrossing history. dailymail