Upshot-Knothole 'Climax' atom bomb test, 1953. Mushroom cloud rising following the detonation of an atomic bomb during testing by the US Army. Flares (left) are being used to measure the blast wave. This nuclear weapons test took place on 4 June 1953, at the Nevada Test Site, USA. Codenamed Climax, it was the 11th test in the Operation Upshot-Knothole series. It was detonated as an air drop, and had a yield of 61 kilotons.