How Two Tunnels Dug From Opposite Ends MEET Perfectly

Two machines dig toward each other from miles apart, deep underground, and meet almost dead center.

With no line of sight and solid rock between them, how do the crews avoid missing by a mile?

It starts at the surface, where surveyors fix the exact position of each tunnel entrance using satellites. That position is carried down the shaft and stretched along the tunnel by a laser fired from fixed points on the wall. The laser hits a target on the machine, so a computer always knows where the head is pointing and how far it has drifted. Because a laser cannot bend around a curve, engineers add a spinning gyroscope that senses true north on its own, correcting the aim over distance. Every few feet, the operator nudges the steering to stay on the line.

Here's the strange part: it works. On the Channel Tunnel, crews boring from England and France met with the tunnel off by only about a foot sideways after tens of miles underground.

Follow The Explainer Vault — because everything has an explanation.