unnel linking Folkestone, Kent in the United Kingdom with Coquelles, Pas-de-Calais near Calais in northern France beneath the English Channel at the Strait of Dover. At its lowest point, it is 75 metres (250 ft) deep.[3][4][5] At 37.9 kilometres (23.5 mi), the Channel Tunnel possesses the longest undersea portion of any tunnel in the world, although the Seikan Tunnel in Japan is both longer overall at 53.85 kilometres (33.46 mi), and deeper at 240 metres (790 ft) below sea level. A tunnel connecting Britain and France has been an engineering holy grail since the early 19th century. The first attempt at a tunnel excavation began in 1880, but the project was abandoned soon after and workings lay dormant for a hundred years. 
In 1988, international treaties were signed and once more excavation work began in earnest.
At 50km from Kent to Normandy, with a full 39km passing 40m under the English Channel seabed, the Channel Tunnel is the world’s longest marine subway.
Eleven tunnel boring machines, working from both sides of the Channel, cut through chalk marl to construct two railtunnels and a service tunnel. The vehicle shuttle terminals are at Cheriton (part of Folkestone) and Coquelles, and are connected to the British and French motorways (M20 and A16 respectively).
Tunnelling commenced in 1988 , and the tunnel began operating in 1994. In 1985 prices, the total construction cost was £4.650 billion (equivalent to £11 billion today), an 80% cost overrun. At the peak of construction 15,000 people were employed with daily expenditure over £3 million. Ten workers, eight of them British, were killed during construction between 1987 and 1993, most in the first few months of boring.
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