Phoenix caissons were parallelepipedal structures made of reinforced concrete, that were submerged in the artificial harbours to complete the blockship breakwaters thereby creating an artificial dyke approximately 2 km from the shore. Six types of Phoenix caissons, varying in length from 53 to 61 m, were constructed in Great Britain. Hollow and equipped with simple bulkheads on the inside, they were floated across the Channel before being sunk off the French coast by filling their compartments using water valves. Some of them (types A and B) were fitted with anti-aircraft equipment.