Well, "the sky" is nearby hills and mountains. This is a flat plane between some hills and some very tall mountains. If you've flown over the area you can see t

Well, "the sky" is nearby hills and mountains. This is a flat plane between some hills and some very tall mountains. If you've flown over the area you can see they even carved a human figure into the side of one of the hills overlooking the plains, as if it was a person standing and observing. Also, it wouldn't be a lot of time to make the figures. It's a plain with an aeolian crust that is easily penetrated. You just need to move some hand-size stones around and scrape the oxidized crust to reveal the lighter substrate. It would probably all wash away after a rain, except it just never rains there.