How do you make terrain have shadows?

I have seen it done before. One map that I looked at had some terrain darker than others, and it was definetely a shadow. When I moved the cliff (that would cause the shadow in real life), the shadow stayed, so I know that it isn't just automatically created. When I edited the shadowed terrain, it stayed darker than others. Even when the shadowed terrain was blighted, it was darker. So, how do you create the shadows and delete them as well?

EZ. Just do: File>Calculate Shadows And Save