Atomic bomb test ANNIE, March 17, 1953. This footage is interesting because the audio is more or less unedited. The timing between seeing the explosion and hearing the blast wave is off by a few seconds, which is how it would be in reality for an observer at that distance (because light travels much faster than sound). All together the audio is great here, so put on some headphones and experience it as if you were actually there.
"In an effort to calm public fears about weapons testing, Annie was an "open shot" -- civilian reporters were permitted to view it from News Nob, 11 kilometers south of the shot-tower. Annie was a weapon development test, it was an experimental device (code named XR3) that provided additional information to normalize the yield-vs-initiation time curve. It was a Mk-5 HE assembly using a Type D pit, and used a betatron for external initiation (the third such test). Total device weight was 2700 lb, predicted yield was 15-20 kt." (http://nuclearweaponarchive.org/Usa/T...)
Video source: US National Archives (http://research.archives.gov/descript...), created by the US Department of Defense (public domain). I have trimmed off some less interesting footage from the original (an interview with a general, some helicopters) as well as a second of the beginning of the tape.