Crater Lake is a caldera lake located in the U.S. state of Oregon. It is the main feature of Crater Lake National Park and famous for its deep blue color and water clarity. The lake partly fills a nearly 4,000 foot (1,220 m) deep caldera that was formed around 5,677 (± 150) BC by the collapse of the volcano Mount Mazama.
Crater Lake is known for the "Old Man of the Lake", a full-sized tree that has been bobbing vertically in the lake for more than a century. Due to the cold water of the lake, the tree has been rather well preserved.
While having no indigenous fish population, the lake was stocked from 1888 to 1941 with a variety of fish. Several species have formed self sustaining populations.
Due to several unique factors, most prominently that it has no inlets or tributaries, the waters of Crater Lake are some of the purest in terms of the absence of pollutants in North America.