The pure, crystalline blue of Crater Lake astounds visitors at first sight. More than 5 miles wide and ringed by cliffs almost 2,000 feet high, the lake was created approximately 7,700 years ago, following Mt. Mazama's fiery explosion. Days after the eruption, the mountain collapsed on an underground chamber emptied of lava. Rain and snowmelt filled the caldera, creating a sapphire-blue lake
so clear that sunlight penetrates to a depth of 400 feet (the lake's depth is 1,943 feet). Today it's both the clearest and deepest lake in the United States—and the ninth deepest in the world.