Underground City, Underground city in Beijing.
The Underground City is a network of tunnels beneath Beijing, China.
Built from 1969 to 1979 for the purpose of military defense during the Sino-Soviet border conflict.
It was designed to accommodate all of Beijing's six million inhabitants upon its completion.
Today, parts of it serve as tourist attractions while others are closed off.
It has been dubbed 'The Underground Great Wall' due to its extensive reach.
Source: Wikimedia