This circular depression on the south polar ice cap is about four kilometers across. It first caught the eye of Mars explorers in the 70s, and they still debate whether it's an impact crater or a collapse feature. The 'Swiss cheese' terrain consists of shallow pits where carbon dioxide ice has sublimated away. This landscape was imaged by the high-resolution camera aboard the Mars Reconnaissance Orbiter on four separate occasions pictured here. From left to right: March 2013, May 2013, September 2007, and December 2012. Click to see at full size.