How were prefabricated caissons used in the Mulberry Harbours during the Normandy invasion?

During the World War II Normandy invasion, prefabricated concrete caissons—specifically known as Phoenixes—were essential components of the Mulberry artificial harbours.

These massive structures were fabricated in secrecy in Britain and then towed across the English Channel to the French coast. Once they arrived at the designated sites off Omaha Beach and Arromanches, the caissons were sunk into position to serve as a primary breakwater. By creating a sheltered area of calm water, these sunken barriers protected the floating Whale roadways and Spud pierheads from the open sea, allowing for the efficient offloading of vehicles and supplies.1

This engineering feat was critical because it enabled the Allies to bypass heavily defended German-held ports, providing a reliable logistical lifeline for the invasion force.1