http://science.nasa.gov/science-news/science-at-nasa/2011/24oct_sleepyhollows/ NASA's MESSENGER spacecraft has discovered strange hollows on the surface of Mercury. Images taken from orbit reveal thousands of peculiar depressions at a variety of longitudes and latitudes, ranging in size from 60 feet to over a mile across and 60 to 120 feet deep. No one knows how they got there.