Largest manmade cave

The "816 Nuclear Plant", located in Chongqing municipality in southwestern China was kept secret when the project began 1966 until construction on the gigantic underground structure was abandoned in 1984. Built in response to China's perception of an increasing threat from the Soviet Union, the Chinese engineers dug up 53 million cubic feet (1,500,792 cubic metres) of dirt from beneath Jianzi Mountain. This resulted in a 13-mile-long facility complete with bridges, roads and cavernous spaces as high as a 20-story building. The facility was supposed to house a nuclear reactor and be able to withstand a nuclear explosion and survive a magnitude-8 earthquake. Today, the facility is open to the public, who can roam its 18 caves connected by over 12 miles of roads.

In building this secret facility, the Baitao Township, where the mountain is located had its population resettled elsewhere and the area was wiped off all maps.