Does anyone know what the Cavernous underground chambers were built for?

Question

behind Malet St.'s ULU in WC1E 7HX opposite the Puritain Hotel/Hostel in Byng Place.

Answer 1

I can only think you mean The Eisenhower Centre.  This was originally planned to be a high speed underground line to ease the Northern Line.  It became an air-raid shelter/command post during the war and was abandoned to become a document storage centre.

http://underground-history.co.uk/shelter...

http://www.subbrit.org.uk/rsg/sites/g/go...

Answer 2

Probably air-raid shelters