In 1969, Chairman Mao commanded the construction of a second Beijing beneath the surface of the original city, designed to accommodate all six million of its then inhabitants, so that if nuclear war did kick off, folk would still have somewhere to hang out and play Mah Jong while the rest of us burnt to death in a shower of atomic rain. War never came, but the city is still there.

http://www.viceland.com/wp/2009/11/chairman-maos-underground-city/