Book 24, Question 80
How do builders dig a tunnel from both ends and meet in the middle?
Surveyors measure everything very carefully, using lasers and satellites to mark the tunnel's exact path. Crews can then start digging from two sides, aiming at the same point. The Channel Tunnel between England and France was dug from both ends, and the two tunnels met with an error of only about one foot. Computers check the direction all along.