Unity's terrain system supports holes. These holes support Physics, Lighting, and NavMesh baking.
These holes have many potential use cases, such as underground caves and tunnels, bottomless world holes, and turning the terrain into a non-rectangular shape.
MM2 doesn't have to support creation of tunnels ro anything beyond creating the holes themselves, but it would be very valuable to be able to leverage the power of MM2's node-graph system in the creation of the holes themselves!