The North Sea is a marginal, epeiric sea of the Atlantic Ocean on the European continental shelf. It is more than 600 miles long and 350 miles wide, with an area of around 222,000 square miles. A large part of the European drainage basin empties into the North Sea including water from the Baltic Sea. The North Sea connects with the rest of the Atlantic through the Dover Strait and the English Channel in the south and through the Norwegian Sea in the north.
The North Sea averages about 100 m (325 ft) deep, with a maximum depth of 700 m (2300 ft) and in some areas shallows can be a mere 15 m deep. The North Sea lies above what used to be the triple junction between three continental tectonic plates in the early Paleozoic Era. Movement on the faults associated with these tectonic phenomena can still cause earthquakes and small tsunamis. The sea's coastal features are the result of glacial movements rather than tectonics. Deep fjords and sheer cliffs mark the coastline of the northern part of the North Sea, whereas the southern coasts consist of sandy beaches and mudflats. These flatter areas are particularly susceptible to flooding, especially as a result of storm tides. Elaborate systems of dikes have been constructed to protect coastal areas.
