Inside a top secret underground Cold War nuclear facility in Chongqing, China

Hidden deep inside lush mountains in the heart of China lies a vast network of tunnels that once once housed a top secret underground nuclear facility – the largest in the world. Construction of the cavernous 816 Nuclear Military Engineering installation began in 1967, three years after China successfully tested its first atomic weapon, and as the Communist state was trying to catch up with the nuclear programmes of the US and the Soviet Union.

Located in the municipality of Chongqing, it was built over a 17-year span by 60,000 soldiers toiling day and night in dangerous conditions. The facility covers 100,000 square metres, the equivalent of 14 football pitches, with a volume equal to 600 Olympic-sized pools. It has the world's largest known network of man-made tunnels, its maze-like corridors extending more than 20km.

The world's largest artificial cave was designed to withstand thousands of tons of explosives and 8-magnitude earthquakes, as well as atomic and hydrogen bombs. The mountainous, fog-covered Baitao Township was chosen as the location for the facility due to the thick forest that surrounded it, so the town was erased from maps.

The facility cost 80 billion yuan (£9.3bn, $11.5bn) but no nuclear material ever passed through it due to a dramatic shift in developments above ground even as soldiers laboured below. China established diplomatic ties with the US in 1979 and tensions with the Soviet Union also eased. Although near completion, the site was judged to have no further use and was abandoned in 1984. Declassified in 2002, it was opened to Chinese tourists in 2010 and began welcoming foreign visitors at the end of 2016.

More than 300,000 Chinese tourists have since visited, while fewer than 100 foreigners had done so as of last month. Surrounded by darkness and damp concrete, visitors are transported back to the era of the Cold War. Just 10% of the corridors, massive halls and control rooms are open to the public. Visitors can tour floors six through nine, which include the central control room and the nuclear reactor, now decorated with fake plutonium bars coloured a luminous green. Visitors can view a light show along with various exhibits, including a model of the first Chinese A-bomb,

The facility's rebirth as a tourist attraction comes as little comfort to the thousands of soldiers who endured hellish conditions in blasting out the site's corridors and halls. "A colleague would detonate the explosives. Then we'd dig away at the rock with a machine. It could have collapsed at any minute," former soldier Chen Huaiwen, now 70, told AFP. Officially, 76 people died in the process, but tour guides and former workers insist that number is too low.

"We'd sleep several to a bed, on straw mattresses," Chen said. "It was a furnace in the summer and you wouldn't get to sleep before 1am. Armed police kept watch outside while we worked on the construction. It was top secret, entry was forbidden. At the time, ordinary people in the area only knew there was some project, they did not know what was being worked on."

The food was basic: rice and beans, with meat thrown in twice a week. "Many got lung problems because of the dust, and that's without taking into account the toxic emissions from explosives, the machine smoke and the foul air," added Chen.

Tears welled up in the eyes of another former soldier who worked on 816, Li Gaoyun, as he viewed old photos displayed in the tunnels. It was his first visit back in 42 years. Li said many of the ex-soldiers who toiled at 816 receive no pensions or welfare benefits despite the enormous sacrifices they made for their country. "A lot of the former workers have no pension, no social security. They don't have enough to live on," Li said. "They owe us that. We gave our blood, our sweat. And our youth."