This picture could be an abstract painting--but those yellow stripes are sand dunes, not paint. This is a picture of Erg Iguidi, a Saharan "sand sea" that stretches from Algeria to Mauritania.

An erg is a sheet of constantly shifting sand--in fact, ergs contain 85 percent of the moving sand on Earth. From far above, Erg Iguidi seems to be composed of wispy, delicate strands of sand, but don't be fooled: its individual dunes can be more than third of a mile high and wide.