Gunnar said there were ships in the sky, meaning clouds, but if you go to Iceland in the winter, you will find whole mountain ranges in the sky, that appear and disappear, created by the mountains out of the wind off the Atlantic.
They’re not exactly shadows and not exactly mirrors. They are amazingly alive. I suspect that the medium (the wind) does that. The image above is near Arnarstapi, on Snæfellsnes. The glacier is just around the corner: one of these clouds that stayed.