Polygonal ridge in Gordii Dorsum Region

This image -- taken from the Gordii Dorsum region of Mars by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter on April 9, 2010 -- shows a large area covered with polygonal ridges in an almost geometric pattern. The ridges may have originally been dunes that hardened through the action of an unknown process. Groundwater may have been involved.