Atomic bomb explosion. Mushroom cloud produced by the detonation of XX-39 Climax, a 61-kiloton atomic bomb, on 4 June 1953. This was one of a series of eleven atomic detonations carried out during Operation Upshot-Knothole at the Nevada Proving Grounds, USA. XX-39 Climax was dropped from the air onto the Nevada desert. Atomic or nuclear bombs work by releasing the energy within the nucleus of atoms. Fission bombs contain enriched uranium or plutonium. When an atom absorbs a neutron, it splits into smaller atoms, releasing energy (in the form of heat and radiation) and more neutrons.