like one winged said, hanging edge is a giant indoor hangar-type structure. it is also located at the point where the hole in Cocoon was blasted hundreds of years again, which lends itself to the name of the area. i think the reason that the whole are has a greenish sort of tint is because the machinery and lighting there is all green, not because the sky is green. the sky in cocoon is very much like our own - blue
as for what all the different structures in there are and what their purpose is...ships maybe, but ya got me.