Following the start of the London blitz the Government decided, in October 1940, to construct a system of deep shelters linked to existing tube stations. London Transport was consulted about the sites and was required to build the tunnels at the public expense with the understanding that they were to have the option of taking them over for railway use after the war. With the latter point in mind, positions were chosen on routes of possible north-south and east-west deep level express tube railways. It was decided that each shelter would comprise two parallel tubes 16 foot 6 inches internal diameter and 1,200 feet long and would be placed below existing station tunnels at Clapham South, Clapham Common, Clapham North, Stockwell, Oval, Goodge Street, Camden Town, Belsize Park on the Northern Line and Chancery Lane and St. Paul’s on the Central Line."
later these were used for storage of highly secret archives
"100ft beneath Holborn there exists a network of tunnels with a fascinating history. Originally built as air raid shelters to provide shelter for up to 8,000 Londoners during the Blitz in 1942, in 1944 they were used as storage by MI6 and in 1945 the then Public Records Office used them to store 400 tons of highly sensitive documents.
During the 1990’s over 150 telephone engineers worked there and had at their disposal amenities such as a snooker hall, canteen and bar. The complex became surplus to BT’s requirements and at the end of last year was offered for sale."