The Strait of Dover or Dover Strait (French: Pas de Calais, , "Strait of Calais", Dutch: Nauw van Calais or Straat van Dover) is the strait at the narrowest part of the English Channel. The shortest distance across the strait is from the South Foreland, some 6 km (4 mi) north-east of Dover in the county of Kent, England, to Cap Gris Nez, a cape near Calais in the département of Pas-de-Calais, France. Between these two points – the most popular route for cross-channel swimmers – the distance is just 33 km (20 mi).
On a clear day, it is possible to see the opposite coastline and shoreline buildings with the naked eye.
Most maritime traffic between the Atlantic Ocean and the North and Baltic Seas passes through the Strait of Dover, rather than taking the longer and more dangerous route around the north of Scotland. The strait is the busiest international seaway in the world, used by over 400 commercial vessels daily. This has made safety a critical issue, with HM Coastguard maintaining a 24-hour watch over the strait and enforcing a strict regime of shipping lanes.
In addition to the intensive east-west traffic, the strait is criss-crossed from north to south by ferries. Until the 1990s these provided the only ground-based route across it. The Channel Tunnel now provides an alternative route, crossing underneath the strait at an average depth of 45 m (150 ft) underneath the seabed.
The strait is believed to have been created through erosion. At one time there was land where the strait is now, a south-east extension of the Weald joining what is now Great Britain to continental Europe. The eastern end of this old longer Weald is the Boulonnais chalk area in the Pas de Calais. The predominant geology on both the British and French sides and on the sea floor between is chalk. Although somewhat resistant to erosion, such erosion of the chalk can be seen on both coasts as impressive sea cliffs, the famous White Cliffs of Dover, and Cap Gris Nez on the French side of the strait. This same rock provided an excellent tunnelling medium for the Channel Tunnel.
The Rhine flowed northwards into the North Sea as the sea level fell during the start of the first of the Pleistocene Ice Ages. The ice created a dam from Scandinavia to Scotland, and the Rhine, combined with the Thames and drainage from much of north Europe, created a vast lake behind the dam, which eventually spilled over the Weald into the English Channel. This overflow channel was gradually widened and deepened into the Strait of Dover. A narrow deeper channel along the middle of the strait was the bed of the Rhine in the last Ice Age. In East Anglia there is a geological deposit that marks the old preglacial northward course of the Rhine.
However, a new study by Gupta et al. (2007) suggests that the English Channel was formed by erosion caused by two major floods. The first was about 425,000 years ago, when an ice-dammed lake in the southern North Sea overflowed and broke the Weald-Artois chalk range in a catastrophic erosion and flood event. Afterwards, the Thames and Scheldt flowed through the gap into the English Channel, but the Meuse and Rhine still flowed northwards. In a second flood about 225,000 years ago the Meuse and Rhine were ice-dammed into a lake that broke catastrophically through a high weak barrier (perhaps chalk, or an end-moraines left by the ice sheet). Both floods cut massive flood channels in the dry bed of the English Channel, somewhat like the Channeled Scablands in the USA.
Many crossings other than in a conventional vessel have been attempted, including by pedalo, bathtub, amphibious vehicle and more commonly by swimming. French law is stricter on such matters than UK law, so most such crossings originate in the UK.