That's gonna be a short post, because all you need to know is over on Robert's¹ blog:

Say you're making a Unity game that takes place in a large landscape dotted with windmills, and some of these windmills have tunnels that lead underground. But in Unity, the terrain collider is generated from heightmap data: it's essentially one giant bumpy plane. You can't punch holes in it.

Can we hack it to achieve the same result? Yep. Here's one way, there are two parts to it:

1. Hiding a piece of terrain geometry with a "depth mask" shader.
2. Disabling the collider so the player...