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 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.
Poole Harbour has an area of approximately 36sqkm. Other large natural harbours are, France's 120sqkm Gulf of Morbihan, New Zealand's 947sqkm Kaipara Harbour and San Francisco Bay in California with a conservative estimate of 1040sqkm covered.