Bouygues Construction played a key part in boring the tunnels from the French side of the Channel Tunnel. Located between France and Great Britain, the Channel Tunnel provides a completely underground 50-kilometre link between the two countries, 37 kilometres of which is under the sea. It was bored at a maximum depth of 40 m below sea level. The cross-channel connection consists of two 7.60 m-diameter single-track tunnels (one in each direction) and a central service tunnel (4.80 m in diameter) for maintenance, ventilation and safety. Both underground routes were designed to allow trains to transfer from one tunnel to the other. At the time, the Channel Tunnel was the longest undersea tunnel to be built using TBMs.