For years I had heard tell of it: A vast underground city beneath Beijing, built back in the Cultural Revolution and able to keep millions of Beijingers safe from nuclear attack. I half suspected that it was an urban myth. I had been in a bomb shelter beneath Peking University, where for a while there was a hip student bar in the early 90s. And the commie chic Red Capital Club kept its wine cellared in a shelter. But until last year I had never actually been...