Lakes are not evenly distributed on the earth's surface; most are located in high latitudes and mountainous regions. Canada alone contains nearly 50% of the world's lakes. Although lakes are usually thought to be freshwater bodies, many lakes, especially in arid regions, become quite salty because a high rate of evaporation concentrates inflowing salts. The Caspian Sea, Dead Sea, and Great Salt Lake are among the greatest of the world's salt lakes. The Great Lakes of the United States and Canada is the world's largest system of freshwater lakes. Lake Superior alone is the world's largest freshwater lake with an area of 31,820 sq mi (82,414 sq km), although there is a larger volume of freshwater in Lake Baykal. The Caspian Sea is the largest lake in the world, with an area of c.144,000 sq mi (372,960 sq km). Lake Titicaca in the Andes Mts. of South America is the world's highest large lake at 12,500 ft (3,800 m) above sea level; the Dead Sea is the lowest at c.1,400 ft (425 m) below sea level.