Elusive, but ecologically vital, Namibia's Ugab River only flows above ground for a few days each year. The subterranean waters underlying this ephemeral river, however, are shallow enough in places to fill hollows and sustain a wildlife population that includes the rare desert elephant.

This image can be found on Landsat 7 Path 180 Row 75, center: 21.7 N, 14.1 E.