816 Project, a decommissioned Cold War-era plutonium and weapons processing facility buried in that mountain in Fuling, Chongqing. It is the largest of its kind in the world and contains the world’s largest artificial cave, while boasting an additional complex of 17 man-made caves and more than 130 roads and tunnels. In all, it spans about 12 miles.

Built in 1966 when Chinese relations with the Soviet Union began to decline. This remarkable piece of history was reputedly home to a factory making plutonium that took 17 years to build, only to be abandoned in 1984 as the Cold War drifted towards its end. Thankfully for the visiting public, it was shut down before the manufacture of plutonium-239 began.

816 project was declassified in 2002 and opened to tourists in 2010.This base has never been put into operation or stored any nuclear material. There is no need to worry about radiation. It is safe to enter. For decades, it functioned partly as a chemical fertilizer factory before being revived for tourism ? an improbable twist of fate for this quirk of Cold War history.

Nowadays, Only a third of 816 is currently open, with other areas of the mountain still highly restricted to tourists. Officials are planning to open other sections in the future.