In June 1944 The Allies invaded Normandy with the intention of liberating France and marching into Germany itself. The Normandy landing was an enormous undertaking needing much organisation, and making sure the troops had enough food and equipment was a big challenge. The Allies might have captured a town with a harbour and dropped off supplies by ships. However, the defences of coastal towns were strong. Many men would be lost taking these fortified towns.
The Allies answered the problem by constructing protective harbours that were made in Britain and then floated in parts to Normandy. They were then reassembled on the Normandy beaches.
The Omaha Beach harbour was destroyed in a storm, but the Arromanches harbour was used for ten months and received over two and a half million troops and four million tonnes of supplies.
In this video we see a drone flying over the site of the Comanches harbor. The caissons, or harbour locks, are the only parts left.