Above is a real piece of WWII history - the remains of one of the concrete sections of what were known as the Mulberry Harbours, used to create artificial harbours to supply the D-Day invasion forces in Normandy, in 1944.
Reputed to have been dreamed up by Churchill, and sketched out on a dinner napkin, the sections were manufactured in secret at several location, but haste meant that not all of them passed the required standards - this one broke its back, whilst being towed to a muster point and has remained in Langstone Harbour ever since.
It now forms a habitat for a variety of wildlife and is more or less "protected".

The inset picture shows a marker post, a few metres outside the Langstone Harbour entrance - the tide is just starting to ebb and a close look at the way the water is surging around the base gives a perfect idea of just how strong and dangerous the current is here.