Did you know? That beneath London exists an underground city spanning 35km of tunnels?
Chislehurst Caves is a 22 miles (35 km) long series of tunnels 100 feet beneath the surface in Chislehurst, Bromley, in the south eastern suburbs of London, England.
Although they are called caves, they are entirely man-made and were dug and used as chalk and flint mines. The earliest mention of the mines is around 1250 and they are believed to have been last worked in the 1830s.
During World War II, when the aerial bombardment of London began in September 1940, the caves were used as an air raid shelter.
http://www.heritagedaily.com/2016/06/did-you-know-that-beneath-london-exists-an-underground-city-spanning-35km-of-tunnels/111601
