The Channel Tunnel (French: Le tunnel sous la Manche; also referred to as the Chunnel) is a 50.5-kilometre (31.4 mi) rail tunnel 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 m (250 ft) deep. At 37.9 kilometres (23.5 mi), the tunnel has 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.

The tunnel carries high-speed Eurostar passenger trains, the Eurotunnel Shuttle for automobiles and other road vehicles—the largest such transport in the world—and international rail freight trains. The tunnel connects end-to-end with the LGV Nord and High Speed 1 high-speed railway lines.

Ideas for a cross-Channel fixed link appeared as early as 1802, but British political and press pressure over compromised national security stalled attempts to construct a tunnel. The eventual successful project, organised by Eurotunnel, began construction in 1988 and opened in 1994. At £4.650 billion, the project came in 80% over its predicted budget. Since its construction, the tunnel has faced several problems. Fires and cold weather have both disrupted operation of the tunnel. Illegal immigrants have attempted to use the tunnel to enter the UK, causing a minor diplomatic disagreement over the siting of the Sangatte refugee camp, which was eventually closed in 2002.

Working from both the English side and the French side of the Channel, eleven tunnel boring machines or TBMs cut through chalk marl to construct two rail tunnels and a service tunnel. The vehicle shuttle terminals are at Cheriton (part of Folkestone) and Coquelles, and are connected to the English M20 and French A16 motorways 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 £12 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.

Initially 38 Le Shuttle locomotives were commissioned, with one at each end of a shuttle train. The shuttles have two separate halves: single and double deck. Each half has two loading/unloading wagons and 12 carrier wagons. Eurotunnel's original order was for nine tourist shuttles.

HGV shuttles also have two halves, with each half containing one loading wagon, one unloading wagon and 14 carrier wagons. There is a club car behind the leading locomotive. Eurotunnel originally ordered six HGV shuttle rakes.

There have been three fires in the tunnel, all on the heavy goods vehicle (HGV) shuttles, that were significant enough to close the tunnel, as well as other more minor incidents.

On the night of 19/20 February 1996, about 1,000 passengers became trapped in the Channel Tunnel when Eurostar trains from London broke down owing to failures of electronic circuits caused by snow and ice being deposited and then melting on the circuit boards.

On 3 August 2007, an electrical failure lasting six hours caused passengers to be trapped in the tunnel on a shuttle.

On the evening of 18 December 2009, during the December 2009 European snowfall, five London-bound Eurostar trains failed inside the tunnel, trapping 2,000 passengers for approximately 16 hours, during the coldest temperatures in eight years. A Eurotunnel spokesperson explained that snow had evaded the train's winterisation shields, and that the transition from cold air outside to the tunnel's warm atmosphere had melted the snow, resulting in electrical failures. One train was turned back before reaching the tunnel; two trains were hauled out of the tunnel by Eurotunnel Class 0001 diesel locomotives. The blocking of the tunnel led to the implementation of Operation Stack, the transformation of the M20 motorway into a linear car park.

The occasion was the first time that a Eurostar train was evacuated inside the tunnel; the failing of four at once was described as "unprecedented". The Channel Tunnel reopened the following morning. Nirj Deva, Member of the European Parliament for South East England, had called for Eurostar chief executive Richard Brown to resign over the incidents. An independent report by Christopher Garnett (former CEO of Great North Eastern Railway) and Claude Gressier (a French transport expert) on the 18/19 December 2009 incidents was issued in February 2010, making 21 recommendations.

A Brussels–London Eurostar broke down in the tunnel on 7 January 2010. The train had 236 passengers on board and was towed to Ashford; other trains that had not yet reached the tunnel were turned back.

In a ceremony presided over by England's Queen Elizabeth II and French President Francois Mitterand, a rail tunnel under the English Channel was officially opened, connecting Britain and the European mainland for the first time since the Ice Age.

The channel tunnel, or "Chunnel," connects Folkstone, England, with Sangatte, France, 31 miles away. The Chunnel cut travel time between England and France to a swift 35 minutes and eventually between London and Paris to two-and-a-half hours.

As the world's longest undersea tunnel, the Chunnel runs under water for 23 miles, with an average depth of 150 feet below the seabed. Each day, about 30,000 people, 6,000 cars and 3,500 trucks journey through the Chunnel on passenger, shuttle and freight trains.