Lake Chad, a freshwater lake located in the Sahelian zone of west-central Africa at the conjunction of Chad, Cameroon, Nigeria, and Niger. It is situated in an interior basin formerly occupied by a much larger ancient sea that is sometimes called Mega-Chad. Historically, Lake Chad has ranked among the largest lakes in Africa, though its surface area varies greatly by season, as well as from year to year. When the surface of the lake is approximately 920 feet (280 meters) above sea level, the area is about 6,875 square miles (17,800 square km); in the early 21st century, however, the area was typically about 580 square miles (1,500 square km). The hydrologic contributions and biological diversity of Lake Chad are important regional assets. The region is noteworthy for important archaeological discoveries, its role in trans-Saharan trade, and its association with historic African kingdoms.