A long-standing mystery Digging deep for a clue to a global mystery

For more than two decades, Yang Shoukai had hoarded his secret, unsure what to do with a possible clue to one of China's most baffling mysteries.

As construction supervisor on the site of an abandoned U.S. military barracks in Tianjin in 1982, he had discovered a strange cement box in the basement of the old wartime barracks. He tried to dig it up, but lacked the proper tools, and the box was buried under a new medical laboratory.

Mr. Yang, now retired and in poor health, is convinced that the box contains a 500,000-year-old archeological treasure that China has hunted for in vain since the Second World War: the missing skulls of Peking Man, one of the most famous links in the evolution of prehistoric humans.