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The beauty and intense blue of Crater Lake have awed people for generations. Five miles wide and ringed by cliffs almost 2,000 feet high, the lake rests in the shattered remnants of a volcano called Mount Mazama, which erupted and collapsed into itself 7,700 years ago. Later eruptions formed Wizard Island and other volcanic features, now hidden under the lake. Crater Lake filled with rain and melted snow. At 1.943 feet deep, it's the deepest lake in the United States and one of the deepest in the world.
Mount Mazama began to grow half a million years ago. Humans have lived around the mountain for more than 10,000 years and probably witnessed its cataclysmic eruption. Crater Lake has long played a significant role in the cultures of local American Indian tribes. In 1853 prospectors in search of gold found the lake, and, as word spread, people began visiting from all over the world. William Gladstone Steel first visited in 1885. Largely as a result of his efforts, Crater Lake became a national park on May 22, 1902.
Scientists have investigated Crater Lake for more than a century. They have plumbed its depths, scaled its cliffs, and explored it with sonar, submarines, and scientific instruments. The eruption that collapsed Mount Mazama was the largest in North America for hundreds of thousands of years. The lake that formed in its shattered remnants is one of the purest and most pristine in the world. The environment provides scientists with an opportunity to study the subtle effects of human influence and global forces.
Today, forests and canyons cover the mountain's outer slopes. Snow blankets the landscape for eight or nine months of the year. Wildflowers bloom late and disappear quickly in the harsh pumice soil, thriving only along streams. Ravens, jays, nutcrackers, and ground squirrels search for seeds along the caldera rim. Deer, elk, black bears, marmots, foxes, porcupines, pine martens, and pikas are present but seldom seen. Visitors wonder about the cataclysmic origins of Crater Lake and marvel at its placid beauty.
--from the NPS brochure (yes, I typed all that out :P )
More great info can be found at Wikipedia.