What look like pale yellow paint streaks slashing through a mosaic of mottled colors are ridges of wind-blown sand that make up Erg Iguidi, an area of ever-shifting sand dunes extending from Algeria into Mauritania. Erg Iguidi is one of several Saharan ergs, or sand seas, where individual dunes often surpass 500 m – nearly a third of a mile – in both width and height. The center and upper right of the enhanced satellite image is black sandstone rock. The light blue and white areas are chalky deposits and salt crusts left behind after water in the area evaporated. Image courtesy of USGS.