Just like the chambers of secret in JK Rowlings Harry Potter Hogwart's school of Witchcraft and Wizardry, You won't believe there's a vast Chambers of Secret 100ft under London. It was hand-built to protect thousands of people from terrifying bombing raids by Nazi Germany during the Blitz in the Second World War.

But the existence of the underground shelter beneath Clapham Common remains a complete secret to the vast majority of Londoners, even those who walk above it every day.
The shelter 'village' lies 100ft below ground and is made up of 1,300 tunnels and is big enough to cater up to 8,000 people, and have now become a special exhibition open to the public.

The tunnels divide into 16 separate shelters and were meant to be used by those who did not have their own in their back garden to protect themselves against Luftwaffe raids.

- See more at: http://www.duketundesblog.com/2016/01/inside-londons-chamber-of-secrets-100ft.html#sthash.Isql4Hyl.dpuf