1) Atomic bomb formation 2) Nuclear explosion aftermath 3) Nuclear weapon 
A mushroom cloud is a distinctive pyrocumulus mushroom-shaped cloud of debris/smoke and usually condensed water vapor resulting from a large explosion. They are most commonly associated with nuclear explosions, but any sufficiently energetic detonation or deflagration will produce the same sort of effect. They can be caused by powerful conventiona...
mushroom (def. 4).