Photograph of a mock-up of the Fat Man nuclear weapon dropped on Hiroshima, Japan, in August 1945. This was the first photograph of the Fat Man bomb casing to ever be released by the U.S. government (it was declassified in 1960). "Fat Man" is the codename for the atomic bomb that was detonated over Nagasaki, Japan, by the United States on August 9, 1945. It was the second of two nuclear weapons to be used in warfare to date, and its detonation caused the third man-made nuclear explosion.