When I look down, sometimes chunks will dissapear, showing the sky. When I look up again, they reappear. I am in a superflat world. I am using a custom preset. It is the normal superflat world, I just added a bunch more layers of dirt so I can have underground areas. It kind of creeps me out. It also happens in normal worlds and the Overworld superflat preset; when I look down in front of a lake.