The Brocken spectre optical illusion happens when a person's own shadow is projected onto a low cloud. The said spectre is often surrounded by a rainbow halo. It is an eerie natural visual effect that used to unnerve mountain climbers. Note that whilst the Brocken happens to be the highest peak in northern Germany, standing at 1,141 metres high, the Brocken spectre effect can also naturally occur on other high, misty mountain peaks. (Click on image to view full size.)