Inverted Crater on Mars National Geographic

It might look like an oddly circular iceberg in the Antarctic Ocean. But this is actually a crater turned inside-out by time in the Arabia Terra region of Mars.
Scientists think such inverted craters form when an impact basin fills with sediment and the material around that sediment gets eroded away.
NASA released the false-color, high-resolution picture March 3, 2010, to commemorate a milestone for the Mars Reconnaissance Orbiter: the collection of a hundred trillion bits of data on the red planet. The camera that snapped this shot on January 29, known as HiRISE, is one of six instruments aboard the Mars-orbiting craft.