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in Poole, Dorset.
Sumbmerged stone structures, once thought to be the remains of a causeway, were investigated by the Poole Harbour Heritage Project. One structure was 160 metres long and 8-10 metres wide, extending from Cleavel Point (on the mainland), the other extending 55 metres towards it from Green Island, with an opening of 70 metres between them. Investigations suggested that the structures were in fact the remains of two harbour piers, of sophisticated construction. Timbers from the mainland pier have been radiocarbon dated to c.300 BC (Iron Age). It is therefore the oldest known constructed port in North West Europe. See Historic England webpage for more information.
The piers are founded on wooden piles, whose ends were sharpened.
Due to the rise in sea level rising relative to the land, the tops of the structures are only rarely visible.