Crater Lake is located in Southern Oregon on the crest of the Cascade Mountain range, 100 miles (160 km) east of the Pacific Ocean. It lies inside a caldera, or volcanic basin, created when the 12,000 foot (3,660 meter) high Mount Mazama collapsed 7,700 years ago following a large eruption.  Crater Lake has inspired people for hundreds of years. No place else on earth combines a deep, pure lake, so blue in color; sheer surrounding cliffs, almost two thousand feet high; two picturesque islands; and a violent volcanic past. It is a place of immeasurable beauty, and an outstanding outdoor laboratory and classroom.

As seen at http://www.nps.gov/