4. The caissons were placed in a continuous row. They were built in six different sizes for different depths of water. The biggest displaced 6,044 tons, the smallest 1,672 tons. Note the sea-cocks used to let in the water to sink them in position. Dredge ships also pumped sand into the compartments. For protection against enemy air attack anti-aircraft guns were mounted on the biggest caissons and barrage balloons floated overhead. Quarters for the A-A gun crews were built on the caissons.