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Poole Harbour is a large natural harbour in Dorset, southern England, with the town of Poole on its shores. The harbour is a drowned valley (ria) formed at the end of the last ice age and is the estuary of several rivers, the largest being the Frome. The harbour has a long history of human settlement stretching to pre-Roman times. The harbour is extremely shallow (average depth: 48 cm), with one main dredged channel through the harbour, from the mouth to Holes Bay.^[1]

Poole Harbour has an area of approximately 36 square kilometres (14 sq mi)^[2] and is claimed to be the largest natural harbour in Europe. Contrary to some claims it is not the largest natural harbour in the world, New Zealand's 947 km² (366 sq mi) Kaipara Harbour and San Francisco Bay in California with a conservative estimate of 1,040 km² (400 sq mi) covered are significantly larger.