Large foggy areas

[The chaser]

I worked once in the Unreal engine. You will surely think this is pointless, but it isn't:

When you made a level where you could see the "deep" or the "blackness" (actually the space where the grid is) the engine turned mad and began to make extra tris and finally crashed. I remember that something similar happened to me once, so you could make a coloured skybox, for your maps sake.

Maybe it works perfectly for you, but maybe other computers don't do what your computer does