So you need to Pick a problem: dig a subway under a city

Dig an open trench, and you'd rip up the street and the buildings along it.

1. So you go underneath, with a tunnel boring machine. Disc cutters crack the rock, and scrapers peel away the soil.
2. But soft ground caves in, and the street above starts to sink.
3. So the soil you dig is packed into a sealed chamber that pushes back on the ground ahead.
4. But to move forward, you need something to push against.
5. So hydraulic rams brace on the lining you've already built, with thousands of tonnes of force.
6. And an erector arm locks curved concrete segments into a ring, and grout fills the gap behind it.

Ring by ring, the tunnel grows, and up on the street, nobody even notices.