A vast "sea" of sand dunes on Mars that surrounds the north polar cap. The dunes are classic forms known as barchan dunes, small, crescent-shaped hills, and transverse dunes, with ridges that resemble coalesced barchans. These dunes are similar in size and shape to familiar sand dunes found in desert regions on Earth. These two varieties form from winds that persistently come from a single direction (in this case, from the southwest).