Danakil Depression, Ethiopia

Discs of travertine ring a 12-foot-wide (3.7-meter-wide) hot spring in Ethiopia's Danakil Depression. Also called the Afar Depression, this area is a tectonic triple junction where the spreading mid-ocean ridges forming the Red Sea and the Gulf of Aden emerge on land and meet the East African Rift. Travertine is a volcanically heated, calcium-rich flow from hot springs.