At one point, Beijing was a walled city. In ’69, for fear of impending Soviet attack (note the era: Cultural Revolution), Mao had them taken down to provide bricks for a network of bomb shelters.

The resulting “underground city” was designed to house 300,000 people for 4 months. Although foreign tourists are allowed to see them, no Chinese are admitted. The little-known entrance is just a bit south of Tiananmen Square.

In a semi-related note, there is continuing construction on the city’s subway, with the junction stations seemingly finished, but the lower levels are in a state of filth and unkept disarray, devoting no effort while they wait for a tunnel that has yet to arrive.