Dover, Strait of, strait, separating Britain from France and the European continent, and connecting the English Channel and Atlantic Ocean with the North Sea. The limits of the strait on the English coast are defined as the promontories of Dungeness and South Foreland; on the French coast, as the promontory of Cape Gris-Nez and the city of Calais. The strait is about 34 km (21 mi) wide. Near the centre of the strait are the Ridge Shoals, more than 13 km (8 mi) long. Both the English and French shores are formed by chalk cliffs. The layers of chalk on both sides show that in prehistoric times a land connection between England and France existed here. The strait is one of the busiest maritime routes in the world, being traversed both by shipping from northern Europe to the Atlantic as well as by ferries connecting Dover and Folkestone with Calais and Boulogne. Proposals were made since the early 19th century to construct a tunnel under the strait. Construction of a joint Anglo-French rail tunnel began in the mid-1980s, and the Channel Tunnel opened in 1994.