The dominant physical structure is a wide basin bounded to the north and east by the Ennedi Plateau and Tibesti Mountains, which include Emi Koussi, a dormant volcano that reaches above sea level. Lake Chad, after which the country is named (and which in turn takes its name from the Kanuri word for lake), is the remains of an immense lake that occupied of the Chad Basin 7,000 years ago. Although in the 21st century it covers only , and its surface area is subject to heavy seasonal fluctuations, the lake is Africa's second largest wetland.