Elusive, but ecologically vital, the 500 km (310 mi) Ugab River only flows above ground for a few days each year. Shown in this false-color satellite image, the subterranean waters underlying this ephemeral river are shallow enough in places to fill hollows and sustain wildlife populations that include the rare desert elephant and the largest free population of black rhino. Pink-granite inselbergs, islands of rock left behind after volcanic activity, form bizarre sculptures in the sandy riverbed. Some, known as "petrified ghosts," are eerie hollow structures, carved by erosion. Image courtesy of USGS.