In the minds of the public, mushroom clouds are generally associated with nuclear explosions. It may come as a surprise that depictions of mushroom clouds have been around since the 1700s.
The term "mushroom cloud" originally came about in 1955 to describe the shape of clouds that rise up after an explosion. There are accounts of clouds shaped like mushrooms from earlier than that, including one describing "a great mushroom of smoke, billowing slowly up some two or three thousand feet [600-900 meters]" during an attack on an Italian ship.
One makes an appearance in a painting of the 1782 Franco-Spanish attack on Gibraltar, while another – likely a weather phenomenon as it came with no prior explosion – was drawn in 1798 of the skies above Gotha, Germany.
Before you concoct a conspiracy that the Franco-Spanish army had nuclear weapons in the 18th Century, nuclear detonations are not the only way that a mushroom-like cloud appears. The clouds are created when a very hot column of gas – likely from an explosion, but not always – shoots up very quickly into the sky. As it cools, the gases begin to rotate in on themselves, causing the mushroom shape. Nuclear mushroom clouds work on roughly the same principle.
The lower portion of the explosion – if it's low enough – draws up dust, debris, and smoke from below, creating the stem of the mushroom clouds, which were common in World War II before the nuclear bombs were ever dropped on Hiroshima and Nagasaki.