How do tunnels not collapse under the immense weight of the soil/mountains?

Question

I get that they’re layered with reinforced concrete but how is it that strong considering the walls aren’t even that thick?

In: 21

Answer 1

The weight of the mountain isn’t only applied straight down, it’s applied to the sides diagonally as well. Like a bridge, the mountain spans over the tunnel just fine.

It’s sort of like jenga, you can remove some of the structure from below without the whole tower falling down.