It is the main feature of Crater Lake National Park and is a tourist attraction for its deep blue color and water clarity.

The lake partly fills a 2,148-foot-deep (655 m) caldera[3] that was formed around 7,700 (± 150) years ago[4] by the collapse of the volcano Mount Mazama.

No rivers flow into or out of the lake; the evaporation is compensated for by rain and snowfall at a rate such that the total amount of water is replaced every 150 years.

Wizard Island, located near the western shore of the lake, is a cinder cone about 316 acres (128 hectares) in size.

Archaeologists have found sandals and other artifacts buried under layers of ash, dust, and pumice that antedate the eruption roughly 7,700 years ago.

"[19][21] Mount Mazama, part of the Cascade Range volcanic arc, was built up mostly of andesite, dacite, and rhyodacite over a period of at least 400,000 years.

The caldera was created in a massive volcanic eruption between 6,000 and 8,000 years ago that led to the subsidence of Mount Mazama.

Also after some time, the slopes of the lake's caldera rim more or less stabilized, streams restored a radial drainage pattern on the mountain, and dense forests began to revegetate the barren landscape.

In the summer, the weather is mild and dry, but in the winter is cold and the powerful influence of the Aleutian Low allows for enormous snowfalls averaging 463.1 inches (11.76 m) per year and maximum snow cover averaging 139 in or 3.53 m. This snow does not usually melt until mid-July, and allows for substantial glaciers on adjacent mountains.

Hard frost is possible even into the summer, and the average window for freezing temperatures is August 20 through July 10, while for measurable (≥0.1 in or 0.25 cm) snowfall, September 28 through June 11.

Clarity readings from a Secchi disk have consistently been measured as being 120 ft (37 m), which is very clear for any natural body of water.

[2] The Klamath people used Crater Lake in vision quests, which often involved climbing the caldera walls and other dangerous tasks.

[42] Swimming is permitted in Crater Lake, but the only legal and safe way to get to the shore is by following Cleetwood Cove trail and people can enter the water from there.

[43] As the region lies within a national park area, collecting rocks within the vicinity is prohibited unless a permit is obtained.

[46] Popular activities within Crater Lake National Park include biking,[47] fishing,[48] cross-country skiing, and snowshoeing.