One of the Mulberry Harbour's Phoenix caissons (breakwaters), 1944
The Mulberry Harbours were specially designed by the Allies for use after D-Day. Troops could be landed much faster than directly onto the beach. The most important point to the Mulberry Harbours was that the pierheads where the ships unloaded were floating, not resting on the bottom - this meant that they could carry on unloading whether it was high tide or low tide.
This is one of the 'Phoenix' caissons, which were sunk in a line to form a breakwater for the Mulberry Harbours. They were huge concrete structures, with many hollow chambers inside. If water was let into the chambers they would sink to the bottom. But if the water was pumped out, they would float, as seen here. In this state they were towed over to Normandy after D-Day. The structure on top is probably an anti-aircraft gun platform.
Reproduction rights: D-Day Museum, Portsmouth. Do not copy. www.ddaymuseum.co.uk