The Channel tunnel ( also called the Chunnel ) is the longest undersea tunnel in the world.; More than 38 km of the tunnel lie under the sea, linking the United Kingdom with France.
The tunnel begins at Folkestone, England, and ends at Sangatte, France. This Engineering marvel actually consists of three tunnels, each 50 km long and bored in the rock below the seabed of the channel. Tow of the tubes are full sized, and accommodate rail traffic.
In between the two train tunnels is a smaller service tunnel that serves as an emergency escape route. There that serves as an emergency escape rote. There are also several cross-over' passages that allow trains to switch from one track one track to another.