A Phoenix caisson under construction.

Repro ID: H4857

Description: One of the huge concrete caissons forming part of the Mulberry harbours. Over 200 were built, with some weighing 5,800 tonnes. Their construction caused a huge security headache for the authorities. Operation Mulberry was top secret, as the building of artificial harbours gave clues as to the likely assault site. The authorities took pains to spread the gossip that these were ‘boom defence units’, designed to protect port installations from German rocket attacks.

Creator: Unknown

Date: 1943-44

Credit line: National Maritime Museum, London