These concrete structures, code named ‘Phoenix’, were designed and manufactured in secrecy in the UK. Hundreds of civilian contractors at locations around the UK coastline worked to create these large pieces that were then placed together to form the Mulberry Harbours. 

​The concrete caissons, ‘Phoenix’, were up to 200ft (61m) long and varied in size from 2,000 to 6,000 tonnes in weight. They were effectively hollow inside and had valves: this controlled water flow into the ‘Phoenix’, and when in the correct position the valves would be opened allowing it to sink into position. 

​​In total 213 ‘Phoenix’ caissons were built, the equivalent length of six miles (nearly 10km). This number was required for the two Mulberry Harbours, as they acted as breakwaters and provided shelter for deep water anchorage. All of them had to be towed across the channel to Arromanches-les-Bains or Veirville-sur-Mer. 

​​‘Phoenix’ caissons were sailed from the UK on 8 June 1944 at 03:30. By 9 June they were being manoeuvred into position and sunk, and by 14 June half of them were in place. The port then began to operate with supplies and vehicles coming ashore. 

​​Here at Arromanches-les-Bains the remains sit off the coast but are reducing in size and condition year upon year due to erosion. There are several still in existence in the UK. The most intact are the two at Portland Harbour, Dorset. Several were transported to the Netherlands to be used in plugging gaps in dykes.​ 

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