One Fine Day
This is a concept map. You start in a house and walk outside to see a big patch of clouds over the roof. Anything existing above the layer of clouds will not be visible to you unless you find a way to actually go up there. Someone could build flying castles and massive airships in the sky and still not be seen until they emerge below the cloud layer for all to see. This is great for ground-vs-air battles and RP alike, there is plenty of space to build and the cloud layer is thick enough to have dogfights inside(though you might want to fly by radar if you have the Wire addon!)
Thanks goes out to Killa-X for the house!
Large map with occlusion layer and fog generators in the sky. Props cannot be seen beyond the occlusion layer from either side, thick fog simulates cloud cover.