Second version

I spent a lot of time designing this version. The main feature I wanted was to allow multiple layers of tunnels which means moving away from simple heightmaps.

I spent a fair amount of time on the idea of having multiple heightmaps but ran into all kinds of problems when trying to seamlessly connect two of them together.

This version uses the idea of heightmaps but does not use a contiguous plane.
Geometry is added with a 'brush' which is a sphere in this case but could be any convex shape. The mesh is quantised in the x and z axis to make the the mesh simple
so I didn't have to deal with splitting polygons. There are still some bugs with this though.