Mulberry Harbour Phoenix Caissons at Portland Harbour

The Mulberry Harbour Phoenix Caissons at Portland Harbour are two 'Phoenix' caissons from 1944, moored end-to-end. Each caisson is constructed from reinforced concrete and weighs 7,000 tons (7,113.8 tonnes).

They are rectangular in shape, measuring 12.19 meters long, 9.14 meters wide, and 12.19 meters high. The caissons have a monolithic appearance, with concrete walls that rise above a wider concrete base, which provides a walkway around each caisson. At the center of each end, there is a vertical concrete pier with an opening that allows passage along the walkway. This pier can be used for ladder access to the roof, and there are fixed ladders at the adjacent ends, along with a gangplank connecting the two caissons.

Steel railings are present at parapet level, and the roofs feature fixings for former 40-mm Bofors light anti-aircraft guns mounted on squat towers. Internally, the caissons are divided into several open transverse chambers that can be flooded to sink them to the sea floor, allowing them to function as a breakwater.