The Channel Tunnel (French: Tunnel sous la Manche, sometimes referred to as the Chunnel) is a 50.46-kilometre (31.35-mile) railway tunnel beneath the English Channel that connects Folkestone in the United Kingdom with Coquelles in northern France. Opened in 1994, it remains the only fixed link between Great Britain and the European mainland.

The tunnel has the longest underwater section of any tunnel in the world, at 37.9 km (23.5 miles), and reaches a depth of 75 m (246 ft) below the sea bed and 115 m (377 ft) below sea level. It is the third-longest railway tunnel in the world. Although the tunnel was designed for speeds up to 200 km/h (120 mph), trains are limited to a maximum speed of 160 km/h (99 mph) for safety reasons. It connects to high-speed railway lines on either end: the LGV Nord in France and High Speed 1 in the United Kingdom.

The tunnel is operated by Getlink (formerly Eurotunnel) and is used by Eurostar high-speed passenger trains, LeShuttle services for road vehicles, and freight trains. In 2017, Eurostar trains carried 10.3 million passengers, freight trains transported 1.2 million tonnes (2.6 billion pounds) of freight, and LeShuttle trains moved 10.4 million passengers in 2.6 million cars and 51,000 coaches, and 1.6 million heavy goods vehicles carrying 21.3 million tonnes (47 billion pounds) of freight. That compares with 11.7 million passengers, 2.2 million cars, and 2.6 million heavy goods vehicles transported by sea through the Port of Dover.

Proposals for a cross-Channel tunnel date back as early as 1802, but concerns over national security delayed development. The modern project was initiated by Eurotunnel in 1988 and completed in 1994, at a final cost of £4.65 billion (equivalent to £11.7 billion in 2023). An engineering marvel, the Channel Tunnel was, at the time of its opening, by far the longest tunnel in Europe, and has only been surpassed by the Gotthard Base Tunnel in Switzerland. However, despite its engineering significance, several economic assessments have found that it has had only a limited positive economic impact on the British economy. Additionally, the tunnel has also experienced occasional service disruptions due to technical faults, fires, severe weather, and unauthorised access by migrants around Calais seeking entry to the United Kingdom.

In 1802, Albert Mathieu-Favier, a French mining engineer, proposed a tunnel under the English Channel, with illumination from oil lamps, horse-drawn coaches, and an artificial island positioned mid-Channel for changing horses. His design illustrated a bored two-level tunnel, with the upper tunnel to be used for transport, while the lower tunnel for groundwater flows.

In 1839, Aimé Thomé de Gamond, a Frenchman, performed the first geological and hydrographical surveys on the Channel between Calais and Dover. He explored several schemes and, in 1856, presented a proposal to Napoleon III for a mined railway tunnel from Cap Gris-Nez to East Wear Point with a port/airshaft on the Varne sandbank at a cost of 170 million francs, or less than £7 million.

In 1865, a deputation led by George Ward Hunt proposed the idea of a tunnel to the Chancellor of the Exchequer of the day, William Ewart Gladstone.

In 1866, Henry Marc Brunel surveyed the sea floor of the Strait of Dover. The results of his survey proved that the sea floor was composed of chalk, like the adjoining cliffs; therefore, the construction of a tunnel would be technically feasible. For this survey, he invented the gravity corer, which is still used in geology.