This is a feature photographed by the Mars Global Surveyor in the Cydonia region of Mars.  It was named the "D&M Pyramid" when seen in earlier Viking photos.  Some people think it is might be an artifact or a natural feature that has been modified artifically.  What do you think?  What natural process might have formed this feature on Mars?  It is not a small feature. It covers several square miles.

Viking Photo of D&M Pryamid

Link to NASA Mars Site