if you wanted to do a tunnel for instance, you could place a piece of level geometry/prefab etc... anything that looked like the entrance to a tunnel on the heightmap. then place an invisible entity with a level change property attached to it in front of the entrance. once the character runs into this invisible trigger, he can be transported to a tunnel level you create in wed (like any other). inside the tunnel level, you can have a similar trigger set up to return to heightmap area..... just a thought
why not put a dip in your hmp map where you and the tunnel, then put a roof on part of it, posibly made of a very small hmp or a block, that might acomplish some sort of underground area. or, you can put a hole where two hmp maps meet by having one dip below the other at one point along the seem.
Re: terrain and hmp#10641 10/12/0218:5010/12/0218:50
it was done very quickly and without consideration of scale or precise placement... it's just to show you that you can actually 'pierce' a heightmap with a piece of level geometry and make it look like a tunnel or sewer pipe or whatever you want. it offers a workaround for what you were discussing.