The current implementation of underground structures already does, so what would we lose?
Interesting. I will check this out. However most LIKELY the underground portions were achieved by basically lowering the terrain and covering it with stones or some other object. Or the terrain was not raised while the "terrain over-ground" was being built with objects, etc. This would be basically the opposite-ish way of handling the same issue, however it would most likely still be somewhat buggy + RIP AI...