Nuclear 'fireball' - definitions 1 or 2.
Source: This image is available from the National Nuclear Security Administration Nevada Site Office Photo Library under number XX-34. This tag does not indicate the copyright status of the attached work. A normal copyright tag is still required. http://en.wiktionary.org/wiki/File:Operation_Upshot-Knothole_-_Badger_001.jpg
Author: Federal Government of the United States