ananyo writes "A series of Martian craters assumed to have been formed by meteorites may actually be extinct volcanoes so massive that, when they were active billions of years ago, they could have buried Mars in ash. The craters pepper the surface of Arabia Terra, a geologically ancient region of northern Mars. They appear as several huge circular pits that resemble Earth's calderas, in which magma beneath a volcano drains after a volcanic eruption, causing the ground above the magma chamber to collapse. Using data from several satellites orbiting Mars, researchers mapped Eden patera in detail. In a report in Nature today (abstract), they describe three separate calderas within the depression, along with possible signs of a lake of solidified lava and a volcanic vent where lava could have oozed out."