The island of Mauritius is volcanic in origin and is almost entirely surrounded by coral reefs. The northern part is a plain that rises to a central plateau, varying in elevation from about 900 to 2,400 feet (270 to 730 metres) above sea level. The plateau is bordered by small mountains that may have formed the rim of an ancient volcano; the highest point (2,717 feet [828 metres]) is Piton de la Petite Rivière Noire in the southwest. The two major rivers, the Grand River South East and the Black River, are the primary sources of hydroelectric power. Lake Vacoas, one of the main reservoirs, is the chief source of water.