This is the Chagan nuclear test. It was part of a larger effort to both test nuclear weapons and to use those weapons for peaceful purposes. The result is a lake it is barely safe to swim in and a severely polluted river nearby.
An often-overlooked aspect of nuclear proliferation was the effort to use nuclear technology for peaceful goals. And when I say "nuclear technology," I don't mean nuclear reactors, I mean nuclear bombs. The United States started Operation Ploughshare, which tested a nuclear bomb's ability to clear roads, empty caverns, and produce hydroelectric power. One of the tests, conducted in 1962, was Storax Sedan. Te Sedan test was meant to explore whether a nuclear bomb could make a crater suitable for a lake. The Sedan Crater is the largest human-made crater in the United States, but as the test done far away from water, it's still dry.
The Soviet Union, in 1965, came up with its own test. The Chagan test was technically underground, but enough radiation escaped to drift east and be detected in Japan. The bomb exploded near the Chagan river, which emptied into the crater and created a new lake. Chagan Lake, the circular splotch above the irregularly-shaped bodies of water in the picture above, was contaminated from the start. Now it is safe enough that swimming in the lake's waters is possible, but not advisable. The nearby river is contaminated as well. These days, tourists do go out to see "Atomic Lake," but the repeated atomic tests in the area have been devastating for the local people.