How are tunnels dug?

Tunnels are dug by cutting into the ground a little at a time, carrying the broken material out, and supporting the new walls before moving ahead. For big routes, engineers often use tunnel boring machines: huge cutters that grind soil or stone, remove the pieces, and set lining rings behind them.

The neat part is how different ground needs different care. Hard rock may be drilled and blasted in a planned pattern, while wet sand or clay may need air pressure and pipes for soupy mud. Two crews can even begin miles apart and meet underground within a tiny margin.