some tipps/hints:
In IFSonly-Setups and with two heightmaps in DEcombination (one heightmapIFS in first and one in second DEcomb Part) u can get a solid/endless ground and so u can cutout – see example – parts of a typical amazingIFS setup and nevertheless the ground.
The smooth look is also a result of RoundedBox with rounded edges.
this „simple“ setup renders comparatively fast – even with reflection (on my pc 8 minutes 3.820 x 2.880)
the „smoky“ glas setup (for this formulas):
Absorbtion: 5
Refractive index: 1.2
Light scattering: 8
no postwork 
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