How did two crews digging from England and France meet under the Channel within 30 centimetres?

Two crews met under the sea within 30 centimetres of each other because they dug through a well-surveyed layer of rock, sent a narrow pilot tunnel ahead, and kept their machines on line with the help of computer-controlled laser guidance systems. Eleven tunnel boring machines, working from both the English and French sides, cut through a layer of chalk marl to build two rail tunnels and a service tunnel. When the two tunnelling efforts finally met, they were misaligned by only 30 cm horizontally and 8 cm vertically, and the whole tunnel turned out to be just 2 cm shorter than estimated.

The idea was an old one, and the first big step was the ground itself. In 1866 Henry Marc Brunel surveyed the sea floor of the Strait of Dover and showed that it was chalk, like the adjoining cliffs, so a tunnel would be technically feasible. The chalk marl is impermeable, easy to excavate and strong. An early attempt was abandoned in 1883 after the British military feared an underwater tunnel might be used as an invasion route.

The tunnel itself has three bores: two 7.6 metre rail tunnels, 30 metres apart, with a smaller service tunnel between them. The service tunnel was used as a pilot tunnel, boring ahead of the main tunnels to determine the conditions, so it was the first to meet. On 1 December 1990, Graham Fagg and Philippe Cozette broke through the service tunnel with the media watching. A BBC commentator called Fagg the first man to cross the Channel by land for 8000 years. Before they met, British tunnellers had picked a Paddington Bear soft toy as the first item to pass through to their French counterparts.

It was not cheap or without danger. Costs at completion were £4.65 billion in 1985 prices, an 80% overrun, and ten workers were killed during construction, most in the first few months of boring.