Caissons off Normandy invasion beaches. Phoenix caissons being put in place as breakwaters for a Mulberry artificial harbour off the Normandy invasion beaches on 14 June 1944. These caissons were built in the UK and towed across the English Channel for use as breakwaters for the Mulberry artificial harbours that were part of the Normandy invasion plan. The D-Day landings of 6 June 1944 were the largest seaborne invasion in history. They successfully liberated the Normandy coast of Nazi-occupied France, contributing to the Allied victory in World War II.