Love the volumetric clouds, 3d trees, sunlight slivers from outside cast on the airplanes when they fly through them, really great scenery details (variations in grass). Is there a game engine that does all this, or did you code it all up from scratch with something like openscenegraph? (I'm one of the original flightgear.org developers so I know how hard these things are to do well and it's just all so seamless you'd hardly notice these things until they were taken away.)