The North Sea, part of the Atlantic Ocean, is the sea between north-western mainland Europe and the island of Great Britain, about 220,000 square miles (570,000km²) in size, but rarely more than about 300 feet (90 metres) deep. It is an important fishing territory and shipping zone, and also has significant petroleum and natural gas reserves.
As well as Great Britain in the west and south-west, the North Sea is bordered by the Norwegian Sea to the north; the Orkney and Shetland Islands (both part of the United Kingdom) in the north-west; Norway to the north-east; Denmark to the east; Germany and the Netherlands to the south-east; and Belgium and France to the south-east. The Strait of Dover and the English Channel connect the North Sea to the Atlantic Ocean south of Great Britain, and the Skagerrek strait links the North Sea to the Baltic Sea.
The coastlines and waterways around and leading into the North Sea have long been historically important because of the development of its maritime trade. The area also forms a natural barrier hindering any attempted invasion of the British Isles. Ports along the fringes of Western Europe saw rapid expansion, and not a few wars have involved fighting over these valuable towns and cities, such as the major siege at Scarborough during the English Civil War, for which the prize was control of the town's port and the North Sea trade routes.