What was the Kazakhstani Lake Chagan created by?

Lake Chagan, Kazakhstan, is a lake created by the Chagan nuclear test fired on January 15, 1965, part of the Nuclear Explosions for the National Economy. A 140 kiloton device was placed in a 178-metre (584 ft) deep hole in the dry bed of the Chagan River. The blast created a crater 400 m (1,300 ft) across and 100 m (330 ft) deep with a lip height of 20 to 38 m (66 to 125 ft).; it is often referred to as "Atomic Lake".