Among the yak herds and Tibetan Buddhism prayer flags dotting the windswept highlands of northwestern China stand the ruins of a remote, hidden city that vanished from the maps in 1958.

The decaying clusters of workshops, bunkers and dormitories are remnants of Plant 221, also known as China’s Los Alamos. Here, on a mountain-high grassland called Jinyintan in Qinghai Province, thousands of Tibetan and Mongolian herders were expelled to create a secret town where a nuclear arsenal was built to defend Mao Zedong’s revolution.