These large blocks on the beach at the West end of Hayling Island Hampshire were anchorage blocks for sections of the Mulbury Harbour called Phoenix caissons.
Hayling Island played its part in WW2 in the construction of the Mulbury Harbour which was constructed prior to D-Day and then towed across the Channel after the invasion to create a serviceable harbour at Aromanche Normandy. Construction of Phoenix Sections was undertaken on Hayling Island. These sections were fabricated on the beach and then launched into the sea where they were submerged until needed. One section broke its back after launch and was towed into Langstone Harbour where it can still be seen. For the invasion of France the Phoenix sections were re-floated and towed to Normandy once in place the caissons were again flooded so the sections sank to form a breakwater in front of the of the floating Mulbury sections.