The shortest distance across the strait, at approximately 20 miles (32 kilometres), is from the South Foreland, northeast of Dover in the English county of Kent, to Cap Gris Nez, a cape near to Calais in the French département of Pas-de-Calais.

[3] This has made traffic safety a critical issue, with HM Coastguard and the Maritime Gendarmerie maintaining a 24-hour watch over the strait and enforcing a strict regime of shipping lanes.

[5] In addition to the intensive north-east to south-west traffic, the strait is crossed from north-west to south-east by ferries linking Dover to Calais and Dunkirk.

It had for many millennia (since the last warm interglacial) been a land bridge that linked the Weald in Great Britain to the Boulonnais in the Pas de Calais.

The Channel Tunnel was bored through solid chalk – compacted remains of sea creatures and marine-deposited, ground up calciferous rock/soil debris.

The first was about 425,000 years ago, when an ice-dammed lake in the southern North Sea overflowed and broke the Weald-Artois (Boulonnais) chalk range in a catastrophic erosion and flood event.

In a second flood about 225,000 years ago supported by glaciers extending from areas then land such as the Zuiderzee, the Meuse and Rhine were ice-dammed into a lake that broke catastrophically through a high weak barrier (perhaps chalk, or an end-moraine left by the ice sheet).

The seabed forms successions of three habitats: The strong tidal currents of the strait at depth slow around its rocky masses as these stimulate countercurrents and deep, calm pockets where many species can find shelter.

This includes the sub-aqueous dunes of Varne, Colbart, Vergoyer and Bassurelle, the Ridens de Boulogne, and the Lobourg channel which provides calmer and clearer waters due to its depth reaching 68 m (223 ft).