How to make Unity ground mix up with sky

I’m building up a scene with Unity, I want the plane to extend far enough and mixed up with sky like the picture showing bellow enter image description here

The only way I can think off in unity is by creating two plane with 90 degree with each other to simulate this scene, I tried to set solid color for the sky but still there is clear gap between the ground plane and the sky, how do you manage to extend the ground far enough and mixed up with sky?