Or the crevice breaks through the ground and into a thickly covered rain-forest. Low and below, as far and high as the eye can see, all is green with tree-cover.
There are no clouds. The definition of the sky included clouds. Therefore, the green cover, (assuming, of course, that they did not realize the plants for what they were,) would not be thought of as the sky.
They break earth in the middle of a cottonfield?