About "Day/Night" little issue

I have a weird problem with the directional light, when the light (sun) reach under the terrain, illumation is shown in terrain as you can see in both pics: Normal Sun above the terrain: http://gyazo.com/fe4067070982793b56c7ccbde1c17692 Sun under the terrain: http://gyazo.com/14169ff97e08f64abc739dee7431d048 As you can see there is light in the terrain, in the trees too, how can I fix this issue? Thanks for read. PS: I tried to put a cube under the terrain but it doesn't work.