An atomic mushroom
Atmospheric nuclear test on Mururoa atoll in French Polynesia. The destructive power of nuclear weapons comes principally from the fireball and wind produced at the moment of the explosion, rather than the residual radioactivity. The majority of the radioactive dust is propelled into the stratosphere, where it will usually remain for a few years. This delay allows for most of the radiation to have disappeared by the time the dust falls back to Earth - accounting for the comparatively weak levels of radioactivity found on the Earth's surface.
CEA/DAM