Around 4,600 BC, catastrophic explosions of lava and ash from Mt. Mazama caused its summit, about 10,800 to 12,000 feet before the eruption, to collapse. The eruption formed Wizard Island and two other submerged cones, as well as a huge crater -- now the deepest lake (1,932 feet) in North America, Crater Lake.
Pictured: Present-day Crater Lake, in January 2006.
More info:
U.S. Geological Survey: Crater Lake, Oregon (Marc Adamus / Associated Press)