This is all that remains of the temporary harbour that was used for the invasion of France on D-Day and beyond. The Mulberry harbour, as it was known, was towed across the English Channel from the U.K. and assembled off the coast of France at Arromanches, also known as Gold Beach. Known affectionately as Port Winston, it survived a terrible storm and lasted several months longer than expected.
65 years later some parts still survive as can be seen here.