I simply can not find a solution for a material, that fades smoothly into the ground, based on the height of the world z axis (Start fading at -500 e.g.).
The upper part of the Mesh should stay complety visibily till the first half and then start fading.
Although you could do something a bit like this in the material, it would mean the material had to be transparent. Which would make the rest of the mesh look weird.
In fact, all of these example have been done just using a fog actor, like the engine exponential height fog.