Photograph of the mushroom cloud of dust, water and radioactive particles produced by the test detonation of an atomic bomb at Bikini Atoll in the Pacific Marshall Islands, on July 1st, 1946 (codenamed: Able Day"). The position of the rapidly-advancing shock wave from the blast is marked by a dramatic change in the texture of the water surface (horizontal line). The explosion was part of a series of atom bomb tests code-named "Operation Crossroads". The photograph was taken by an automatic camera situated on an island some miles away from the blast centre."