The 1940 Blitz - and the consequent pressure to allow people to use Tube stations for shelter - jolted the Government into authorising the construction of 10 deep shelters, each holding 8,000 people.
Two were not completed - at St Paul's because of concern that construction would damage the cathedral, and at Oval Tube Station because of flooding. The ventilation shaft at the Oval is now used as the emergency staircase.
The other eight were at Belsize Park, Camden Town, Goodge Street, Chancery Lane, Stockwell, Clapham North, Clapham Common and Clapham South. All are on the Northern Line apart from Chancery Lane which is on the Central Line.
Each shelter was built in tunnels very similar to the existing Tube tunnels. Each consisted of shelters on two levels, each containing two parallel tubes - so making four tubes in total - all running underneath the existing Tube lines. (In a clever piece of planning, it was hoped that the tunnels might one day be used to run a new railway through London.) Each tube/shelter was 400 metres long divided into four areas, each capable of accommodating 500 people. All were deep underground - Clapham South is 37 metres down.
It took c. two years to complete the shelters by which time they were not needed until V1 and V2 bombs started raining down. After the war they were used as very cheap hotels, often used by school groups etc., for instance visiting the Festival of Britain. The Clapham South shelter was used for a few weeks by some of those arriving from Jamaica on the Festival Windrush. Their nearest Job Centre was in Brixton, which is why so many West Indians then settled in that area.
For many years from 1956 the shelters were used for secure storage. Two (Goodge Street and Stockwell) are still used for that purpose, and salad crops are grown hydroponically at Clapham North and Clapham Common. Clapham South is occasionally opened to the public by London Transport, and may one day be opened as a museum.
Interesting fact:- The Central Line in East London - under construction as the war began - was temporarily used as a munitions factory. Here is a 2017 Tweet from the London Transport Museum: