How do you set "ground level" in a skybox?

[Ivy Norsk]

I adjust these sorts of things by putting in extra invisible prims. Also, I rarely let teacups run around in any case; I'm afraid of them disappearing into the floor or getting caught under rugs and such. There are many more problems with mesh floors. A good solid prim helps a lot. Sometimes a strategic prim rug on top of mesh is just the ticket.