The Phoenix breakwaters were a set of reinforced concrete caissons built as part of the artificial Mulberry harbours that were assembled as part of the follow-up to the Normandy landings during World War II. They were constructed by civil engineering contractors around the coast of Britain. They were collected at Dungeness and Selsey, and then towed across the English Channel and sunk to form the Mulberry harbour breakwaters replacing the initial 'Gooseberry' block ships.[1] Further caisson were added in the autumn of 1944 to re-enforce the existing structure to cope with the harbour continuing in use longer than planned.[1]
Several Phoenix breakwaters are still in use in Britain: two are part of the harbour off Castletown at Portland Harbour and two can be dived in less than 10 metres of water off Pagham. There is also a smaller Phoenix Caisson (type C) in Langstone Harbour.[1]
Four Phoenix breakwaters were used in the Netherlands to plug a gap in the dyke at Ouwerkerk after the North Sea Flood of 1953. They have now been converted into a museum for the floods called the Watersnoodmuseum. You can walk through the four caissons.