A mushroom cloud characteristic of an atomic bomb detonation.
Source: http://www.archives.gov/research/military/ww2/photos/images/ww2-163.jpg National Archives image (208-N-43888). http://en.wiktionary.org/wiki/File:Nagasakibomb.jpg
Author: The picture was taken by Charles Levy from one of the B-29 Superfortresses used in the attack.