The Channel Tunnel, also called the Chunnel, is a rail link between Folkestone in England and Coquelles in France, near Calais.
It runs 31.4 miles (50.5 kilometers) in all.
23.5 miles (37.9 kilometers) of that lie under the water.
That is a record: the longest undersea stretch of any tunnel in the world.
The two outer tubes carry trains. The middle tube is for maintenance and safety. The rail tunnels are high enough for a double-decker bus.
At the lowest point, 250 feet (75 meters) of rock sits above the ceiling.
That is all that stands between the rails and the seabed.
Eleven huge boring machines start from the British coast and the French coast. For years they cut through the rock under the strait.
On December 1, 1990, the two halves of the service tunnel meet.
The gap?
Just 14 inches (35.8 centimeters) to the side and 23 inches (58 centimeters) in height, after miles of work in the dark.
Under the sea there are also crossovers: caverns as wide and as long as a cathedral, so a train can switch from one tube to the other.
On a map the English Channel looks like a narrow strait.
Beneath it lies the longest underwater path in the world.
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