Mercury continues to surprise and confound scientists as a result of the ongoing NASA MESSENGER mission. The latest revelation involves newly discovered features called "Sleepy Hollows" - pretty scientific sounding term I must say.
According to NASA, "A key clue is that many of the hollows are associated with central mounds or mountains inside Mercury's impact craters. These so-called “peak rings” are thought to be made of material forced up from the depths by the impact that formed the crater. Excavated material could be unstable when it finds itself suddenly exposed at Mercury's surface."
Be sure to read more about it: http://science.nasa.gov/science-news/science-at-nasa/2011/24oct_sleepyhollows/
Sky Guy in VA