Charles Levy, 1945. Mushroom cloud taken from a B-29 follow-on aircraft — the second atomic bombing.
From Wikipedia
A mushroom cloud is a distinctive mushroom-shaped flammagenitus cloud of debris, smoke, and usually condensed water vapour resulting from a large explosion. The effect is most commonly associated with a nuclear explosion, but any sufficiently energetic detonation or deflagration will produce a similar effect. They can be caused by powerful conventional weapons, including large thermobaric weapons. Some volcanic eruptions and impact events can produce natural mushroom clouds.