Operation Crossroads was a series of nuclear weapon tests conducted by the United States at Bikini Atoll in the summer of 1946. Its purpose was to test the effect of nuclear weapons on naval ships. The series consisted of two detonations, each with a yield of 23 kilotons: Able was detonated at an altitude of 520 feet (158 m) on 1 July 1946; Baker was detonated 90 feet (27 m) underwater on 25 July 1946. A third planned burst, Charlie, was cancelled.
The Crossroads tests were the fourth and fifth nuclear explosions conducted by the United States (following the Trinity test and the bombings of Hiroshima and Nagasaki). They were the first nuclear tests held in the Marshall Islands, and the first to be publicly announced beforehand and observed by an audience of invited witnesses and a large press corps.
Ultimately, the biggest news from Crossroads, not widely reported at the time, was the radioactive contamination of all the target ships by the Baker shot. It was the world's first experience with immediate, concentrated local radioactive fallout from a nuclear explosion. (The global fallout from an air burst is delayed and widely dispersed.)
The most famous Baker picture. The Wilson cloud has lifted revealing the fully formed spray column, with the upended battleship Arkansas, right side, falling back into its northern perimeter.