deep, clear, intensely blue lake within a huge volcanic caldera in the Cascade Range, southwestern Oregon, U.S., about 50 miles (80 km) northeast of Medford. The lake and its surrounding region became Crater Lake National Park in 1902, with an area of 286 square miles (741 square km).

The crater from which the lake was formed, about 6 miles (10 km) in diameter, is the remnant of Mount Mazama, a volcano that rose to probably 12,000 feet (3,700 metres) until an eruption about 7,000 years ago destroyed the upper portion. Subsequent lesser outbreaks are indicated by cinder cones on the caldera floor; one of these, Wizard Island, rises 764 feet (233 metres) above the water. Crater Lake has an average surface elevation of 6,173 feet (1,881 metres) above sea level and an average depth of about 1,500 feet (457 metres). Underwater mapping of the lake in 2000 established a maximum depth of 1,943 feet (592 metres)—the previous recorded maximum had been 1,932 feet (589 metres)—making it the deepest lake in the United States and the seventh deepest in the world. Its waters are exceptionally clear, and it is often possible to see to a depth of more than 100 feet (30 metres).