816 Nuclear Military Plant

The 816 Nuclear Military Plant is an unfinished Chinese underground nuclear weapons production facility located near suburban Fuling in Chongqing Municipality. Once the world's largest man-made tunnel structure, this Cold War-era megaproject was designed to house a nuclear reactor and support facilities deep within limestone karst caves extending over 2 kilometers underground. The complex includes 17 artificial caves and a massive reactor hall, making it the largest artificial cave system in the world. Construction began in the 1960s during heightened tensions with the Soviet Union but was halted in 1984 before completion.