This is an amazing effect you can use to mimic the Source engine's 3D skybox in any Goldsource game. In essence, what this little trick does is replace a massive amounts of world geometry with a skybox model with an origin in the center of the map. Since the model uses a significantly smaller amoutn of CPU to render than its world-brush equivalent, you can have a seemingly endless world without killing your PC.
The only downside to this is that you will have to find some way for the origin of the skybox model to ALWAYS be visible, but where there's a will there's a way. Put it in a reversed hint box or something.