Roundhead of St. Catherine's breakwater. Work began on the breakwater in 1847 when Britain was fighting the Napolini wars and was completed with the construction of the large round end in 1855.
It was constructed by tipping a huge mound of rubble - a bund - on to the sea floor off the end of a wooden trestle.
On to this the walls of dressed stone blocks with rubble infill was built.
The plan was to have this northern breakwater accompanied by a southern one to create a huge "refuge" harbour from which the Royal Navy could challenge the French Fleet as soon as they left harbour.
By 1855 Britain and France had become allies against the Russians and the harbour was no longer needed.
The roundhead is some 606m out to sea and, after 150 years of buffeting by the sea, is beginning to subside.
It is thought the bund was not large enough and cavities have formed in the rubble inside the roundhead.
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