The Channel Tunnel (French: Le tunnel sous la Manche; also referred to as the Chunnel) is a 50.5-kilometre (31.4 mi) undersea 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 metres (250 ft) deep. 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.

The tunnel carries high-speed Eurostar passenger trains, Eurotunnel Shuttle roll-on/roll-off vehicle transport—the largest 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. In 1996 the American Society of Civil Engineers identified the tunnel as one of the Seven Wonders of the Modern World.

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. The project came in 80% over its predicted budget. Since its construction, the tunnel has faced several problems. Fires have disrupted operation of the tunnel. Illegal immigrants and asylum seekers 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.