terrains are a tricky beast. The way it would work is if you build your terrain out of different BoxColliders, CircleColliders and PolygonColliders (keep in mind the maximum number of vertices is 8).
So in your case (“digging”) it sounds like you could do what you want with BoxColliders. Performance can be improved if you put the lower boxes in your terrain to sleep so they don’t affect the physics calculations.
This property holds whether the body is allowed to get inactive for performance optimizations. The default value is the inverse of collisionTestingOnlyMode, so <tt>sleepingAllowed: !collisionTestingOnlyMode</tt>. This is needed to avoid sleeping when the collider gets used for collision detection, because if no forces or impulses are applied to the body it will sleep by default.
If collisionTestingOnlyMode is left at its default value (Which is false), this property has the default value <tt>true</tt>.