Project 6501—’The Underground Great Wall,’ the largest unfinished military project in China?

This is a part of declassified historical data—the mysterious project “6501” comes from document No. 1 of the Central Military Commission of the Communist Party of China (CCP) led by Vice Premier and Minister of National Defense Lin Biao promulgated in 1965. This vast underground project cost over $86 million (600 million yuan), was secretly built by the military, and was dubbed the mysterious “Underground Great Wall of China.” Recent reports have revealed the mystery of this massive man-made structure.