AFAIK, ZH doens't even have a "gravity" for most activies, the gravitational force only applies during physics calculations, which, afaik, only applies to moving vehicles, not infantry or buildings
As for subterreanian stuff, its probably just a unit animation that causes it to "go under" whilst a few polys are visible above the ground surface (so you can select it).... it probaby has the "stealth whilst moving" thing applied too.
But what would be cool is a map that hides the main ground layer and everything above it when you click on something, then showing you another ground layer below it with all the subterranian units, now *that* would be cool
