"Fat Man" was the codename for the type of atomic bomb that was detonated over the Japanese city of Nagasaki by the United States on 9 August 1945. It was the second of the only two nuclear weapons ever used in warfare, the first being Little Boy, and its detonation caused the third man-made nuclear explosion. It was built by scientists and engineers at Los Alamos Laboratory using plutonium from the Hanford Site and dropped from the Boeing B-29 Superfortress Bockscar. For the Fat Man mission, Bockscar was piloted by Major Charles W. Sweeney.