The country's most marked feature is Lake Chad, which is situated at the foot of a gently sloping plain and is surrounded by vast marshes. Fed chiefly by the Chari and Logone rivers. The surface area of the lake varies from about 9,842 to 25,641 sq km (3,800–9,900 sq mi). From this low point of 230 m (750 ft) above sea level, the land rises to a maximum of 3,415 m (11,204 ft) at Emi Koussi, an extinct volcanic peak in the Tibesti Mountains of northern Chad. The center of the country is primarily a shallow bowl known as the Bodélé Depression.