816 Nuclear Military Plant

816 Nuclear Military Plant is an unfinished Chinese underground nuclear weapons production facility and the largest man-made tunnel structure in the world. A military megaproject, the nuclear base is located near what is now suburban Fuling, a municipality in Chongqing, China. In 2010, it was opened to Chinese tourists. It is a distinct network of nuclear-weapons manufacturing tunnels to the likewise defunct Underground Project 131 and the still operational "Underground Great Wall of China."

Source: Wikipedia