This was no minor undertaking. In the late 1960s, the population of Beijing reached 7.5 million residents [source: CPIRC]. In short order, the residents of the capital city were put to work excavating their enormous air raid shelter. Most of the digging was done by hand, and the work was shared by adults and schoolchildren alike. This communal venture fit nicely into Chairman Mao's Cultural Revolution -- a massive campaign to support the communist movement and thwart counterrevolutionary ideas. From 1969 to 1979, the people of Beijing focused their attention underground.
When the Dixia Cheng (underground city) was completed, it was capable of housing 300,000 people for about four months [source: Time]. Between 26 to 60 feet (8 to 18 m) beneath the city, tunnels stretch about 18 miles (approximately 30 km) in length and spread over a more than 52 square mile area (85 square km) [source: Zhiyong]. Ancient city gates throughout the city were recycled into construction material for the tunnels. Secret entrances aboveground were located in shops, homes and parks around Beijing. A map of the tunnels drawn using a fluorescent medium to render it invisible to the naked (Soviet) eye was found on the wall of one of these shops [source: Lonely Planet].
The massive bomb shelter complex was never used for its intended purpose. Had the occasion arisen, the Beijing residents who made it underground wouldn't have died easily. The Dixia Cheng is outfitted with ventilation shafts that resist fallout from nuclear, biological and chemical (NBC) weapons. In addition to the safety provided by the depth of the tunnels, the complex is outfitted with bomb shelters. Other chambers in auxiliary tunnels held grain, weapons and other supplies. The underground city also has sites for growing sunless crops like mushrooms and areas prepared for well drilling [source: Zhiyong].
Subterranean dwellers wouldn't have died of boredom, either. Classrooms were constructed for the children living in the underground city, and amenities found aboveground like a movie theater, barber shops, restaurants and a roller skating rink were all awaiting the flood of Beijingers seeking shelter underground from Soviet bombs.
After the USSR disintegrated, and Beijing was no longer under the threat of attack, the underground city lost its purpose. Its existence faded into obscurity and conjecture. In 2000, the Dixia Cheng found new life as a tourist destination (it's inexplicably not open to Chinese visitors). Most of the complex is shut off; a mere fraction of it is open to tours, and some businesses have set up shop in the open areas. A few air raid shelters are now hostels for thrifty travelers. Urban explorers' unsanctioned investigations of the untouched portions of the tunnels have yielded reports of a labyrinth frozen in time: posters of Chairman Mao still adorning walls of rooms where bunk beds stand silently [source: CNN].
As the 2008 Olympics approached, Beijing officials reinvigorated districts in the city. This included tearing down some of the shops that housed the most well-known entrances to the underground city. Some are fearful that the Dixia Cheng will be lost forever: Without entrances and the luminescent map to show the way, Beijing's Dixia Cheng may return to its shroud of secrecy.
taken from: http://geography.howstuffworks.com/asia/beijing-underground-city1.htm
