It is well-known that during the heavy bombardment of World War II, Londoners took to the Underground in droves, using the stations to shelter from German blitz attacks. Beneath the ground, people ate and slept in relative safety. Also, early in the war, the underground was used to transport 200,000 children from the city centre to the safer suburbs.
At first, the London Transit authority tried to discourage people from sheltering in the stations, in order to keep trains clear for evacuating the dead. However, when notices were posted that stations were not to be used as air raid shelters and that only passengers would be permitted, people bought tickets and went down anyway.
Soon London Transit had matters organized and being underground became a way of life. LT issued tickets to regular users, added bunk beds and toilets and ran special trains to deliver tons of food and thousands of gallons of tea and cocoa. The London County Council got involved and established libraries in most of the 79 station shelters. There were even musical evenings, movies and dances underground. Naturally there were medical personnel stationed below -- 36 doctors and 200 nurses by then end of the war. (Long, 2009)
Overall, the Tube proved to be a good bomb shelter. Amazingly few were killed,
though there were some casualties. A bomb at Bank in 1941 killed 56
people and wounded 69 when the station collapsed, leaving a crater so
large that a Bailey bridge was temporarily constructed over the
opening. Marble Arch was also hit, killing twenty, and so was Bounds
Green, where ironically, most of the victims were refugees who had
escaped Dunkirk. In 1943, someone tripped on the stairs at Bethnal
Green, causing others to fall. This resulted in 173 deaths of men, women
and children, who are commemorated by a plaque at the station.
In 1940, a decision was taken to bore a new system of tunnels, linked to the Tube, and a plan was made to construct a system of deep level shelters under 7 stations. Four were designed for civilians, but the others were for government use. As the Allies prepared for D-Day, General Eisenhower had his London headquarters deep under Goodge Street Station. This facility was directly linked by pneumatic tube (for message delivery) to Churchill's Cabinet War Rooms under Whitehall.
The deep underground system was never completed due to postwar austerity measures. However, good use was made of the spaces below the city. In 1948 Clapham Common was used to temporarily shelter several hundred Commonwealth citizens who arrived on the SS Empire Windrush, and beneath Chancery Lane, the Public Record Office stashed tons of top secret documents.