Only thirty-three kilometres of water keep Britain from the Continent
At its narrowest the Strait of Dover spans just 20.6 miles—33.2 kilometres—between England and France, the English Channel’s tightest pinch before the North Sea. On clear days the White Cliffs are visible from the French shore with the naked eye.
Most Atlantic–North Sea and Baltic shipping threads this corridor rather than circling Scotland, making it one of the world’s busiest international seaways. Ferries still cross northwest–southeast between Dover, Calais, and Dunkirk; until the Channel Tunnel opened in 1994 they were the only non-air link. Dover lends its name to a Shipping Forecast sea area. The Lobourg trench cuts a six-kilometre-wide slash to 68 metres depth nearer France, while banks shoal to about 20 metres.
For millennia after the last warm interglacial a chalk land bridge tied Britain’s Weald to France’s Boulonnais. A 2007 study by Sanjeev Gupta and Jenny Collier argued two catastrophic floods carved the strait: an ice-dammed southern North Sea lake overflowed about 425,000 years ago, then a second megaflood finished the job. Rising seas drowned Doggerland around 6,500–6,200 BCE, sealing Britain as an island. Pleistocene ice had dammed the Rhine–Thames system into a lake whose spillways helped scour the gap.
More than 400 commercial ships pass each day, so HM Coastguard and France's Maritime Gendarmerie keep a round-the-clock watch and enforce strict lanes; although every metre lies in British or French territorial waters, foreign vessels enjoy a right of transit passage under the UN Law of the Sea. Below, the Channel Tunnel runs through solid chalk an average of 45 m beneath the seabed. Rocky patches, avoided by trawlers protecting their nets, create calm, clear pockets where algae grow and some endemic species shelter, and about 682 km² is protected as a Natura 2000 zone. Swimmers favour the South Foreland to Cap Gris-Nez line, and odder attempts by pedalo, jetpack and bathtub mostly start in England because French law forbids many of them. In the Little Ice Age winter of 1684, ice reportedly left only a league of open water between Dover and Calais.
Source: Strait of Dover
