The dried crust of the salt lake Assale in the northn Danakil depression in Ethiopia near the Eritrean boundary. The salt here is several kilometers thick and the result of larg-scale evaporation of sea water when the area now lying at 130 m below sea level had access to the Red Sea. The deep trench, or graben, where the lake is located was formed by giant tectonic forces ripping Africa apart along the Rift Valley. (Photo: Tom Pfeiffer)