The Channel Tunnel is a 50.45-kilometre (31.35 mi) railway tunnel that connects Folkestone (Kent, England) with Coquelles (Pas-de-Calais, France), beneath the English Channel at the Strait of Dover. 

It is the only fixed link between the island of Great Britain and the European mainland. At its lowest point, it is 75 m (250 ft) deep below the sea bed and 115 m (380 ft) below sea level. At 37.9 kilometres (23.5 mi), the tunnel has the longest underwater section of any tunnel in the world. 

Plans to build a cross-Channel fixed link appeared as early as 1802, but British political and media pressure over the compromising of national security had disrupted attempts to build a tunnel. 

An early attempt at building a Channel Tunnel was made in the late 19th century, on the English side, "in the hope of forcing the hand of the English Government". The eventual successful project, organised by Eurotunnel, began construction in 1988 and opened in 1994. 

Valued at £5.5 billion in 1985, it was at the time the most expensive construction project ever proposed. The cost finally amounted to $9 billion (equivalent to £16 billion in 2016), well over its predicted budget. 

According to en.wikipedia