Earlier this week I had the chance to look around on of the deep level shelters constructed during World war Two.
There are several excellent websites that give details of the history of these shelters
This is a brief extract from the Underground History site which explains the origins of the shelters:
As congestion on the Northern Line increased in the '30s, a plan was
developed to build a second pair of tunnels in parallel with the Charing
Cross branch of the Northern Line that would act as an express route
through London. These plans were shelved at the outset of the Second
World War, but as the platforms of the Underground became increasingly
used by the general public overnight as air raid shelters (despite being
initially discouraged), work began in 1940 on building deep level
shelters which were envisaged to eventually become the platform tunnels
for the express route.
Ten shelters were originally planned - five to the north of the
Thames and five to the south. All of these were to eventually form part
of the Northern Line express route apart from Chancery Lane and St.
Pauls, which were associated with the Central Line. The stations on the
Northern Line that were equipped with extra tunneling are: Belsize Park,
Camden Town, Goodge Steet, Stockwell, Clapham North, Clapham Common,
Clapham South. Work on St. Pauls was abandoned in 1941 for fear of
damaging the cathedral's foundations and also work ceased on a shelter
at Oval soon after, due to extensive flooding. It is unknown whether
Oval would have been included in the express route.
Above ground, each shelter's shafts were protected by specially
constructed 'pill box' buildings to prevent any bombs that directly hit
the location from going underground. Each pill box housed lift machinery
and provided the cover for spiral staircases down to the shelter's
Below is the northern entrance to the Clapham Common Shelter
Each shelter consisted of two parallel tunnels that were 16ft 6in
(approx. 4.9m) in diameter and were 1400ft (approx. 427m) in length. Two
pairs of shafts were sunk for each shelter, with the pairs being sited a
distance from each other in case a bomb struck, blocking a shaft. At
each location, one shaft was for the spiral staircase and lift, the
other a narrower ventilation shaft.
The two tunnels were interconnected at various places along their
length. A floor was constructed at the horizontal diameter level of the
tunnel, providing two decks of accommodation.
A plan of the tunnels was on the wall showing fire points for the document storage company which had occupied the tunnels.
A plan on the Subterranea Britannica site is similar:
The precast concrete planks which form the mid-height floor are supported on steel beams with a timber packer!
There was very little evidence of their use as a WW2 shelter apart from this sketch on one of the walls which looks as if it might date back to the 1940's