As the Germans had turned all the Channel ports into fortresses, there was no question of the Allies capturing them in an all-out attack from the sea.
And yet they need harbour infrastructures in order to land the impressive quantities of men, material and munitions they needed if Operation Overlord was to succeed.
Hence the idea – mooted by Winston Churchill himself – of using artificial harbours. Their different components would be built in England and towed across the Channel for assembly in front of the Normandy coast.
In addition to the platforms on stilts needed to unload the ships out at sea and the floating roadways for carrying the supplies to the shore, the most spectacular feature of the Mulberry project was, without, doubt the construction of the huge, hollow blocks of concrete, or Phoenix caissons, to form the roadstead. Before being flooded, they each weighed between 1,600 and 6,000 tonnes, while the largest ones measured sixty metres by seventeen, and were the height of a five-storey building.
Two Mulberries were built, one in the American sector, opposite Omaha, the other in the British sector, opposite Arromanches.
A total of 40,000 workers were involved in this gigantic project, which required the opening of special building sites across England.