This is interesting, a recent HiRISE photo from the Mars Reconnaissance Orbiter spacecraft showing an oval pit or crater with an opening in the bottom (cropped here from one of the larger images) near Galaxias Chaos on Mars. The opening is also oval, and you can see some sand dunes on the bottom. How did it form? More images are available here.
Paul Scott Anderson is a freelance space writer with a life-long passion for space exploration and astronomy. He currently writes for The Spaceflight Group, AmericaSpace and Examiner.com. His own blog The Meridiani Journal is a chronicle of planetary exploration.