How are tunnels dug?

Big tunnels are usually made by a huge boring machine that presses its spinning cutter into earth or stone, breaks the ground apart, and carries the pieces away. As it moves, workers add a strong lining behind it so the new passage does not cave in.

In softer ground, the machine has to keep pressure at the front so wet soil stays put instead of flowing inward. Some tunnels are dug from both ends at once, with careful measuring helping the crews meet deep underground.