This oddly circular outcrop has been discovered on Mars. Scientists don't know exactly what the 2km wide feature is — although it appears to be surrounded by mass of smooth, solidified lava flows.
Snapped by HiRISE aboard the Mars Reconnaissance Orbiter, NASA speculates that "perhaps lava has intruded underneath this mound and pushed it up from beneath. It looks as if material is missing from the mound, so it is also possible that there was a significant amount of ice in the mound that was driven out by the heat of the lava." But they don't really know. Any guesses? [NASA]