Crater Lake lies inside a caldera, or volcanic basin, created about 7,700 years when the 12,000 foot (3,660 meter) high Mount Mazama collapsed following a major eruption. The lake averages more than 5 miles (8 km) in diameter, and is surrounded by steep rock walls that rise up to 2,000 feet (600 meters) above the lake’s surface. The lake itself is 1,943 feet (592 meters) deep at its deepest point, the deepest lake in the United States and the seventh deepest anywhere in the world. It is fed almost entirely by snowfall, which averages 533 inches (1,354 cm) per year. There is no outlet to the lake; evaporation and seepage prevent the lake from becoming any deeper.