Gabon's largest river is the Ogooué which is 1200 km long. Gabon has three karst areas where there are hundreds of caves in the dolomite and limestone rocks. Some of the caves include Grotte du Lastoursville, Grotte du Lebamba, Grotte du Bongolo, and Grotte du Kessipougou. Many caves have not been explored yet. A National Geographic Expedition visited the caves in the summer of 2008 to document them (Expedition Website).