In 1957, a hydrogen bomb fell out of an Air Force bomber near Albuquerque, New Mexico. Turbulence had caused the navigator to grab the bomb release mechanism to keep from falling. The nuclear portion of the bomb didn’t explode, but the conventional portion did, creating a crater twelve feet deep and twenty-five feet across. The only casualty was a cow.
The Air Force wouldn’t confirm this accident until 1981, when they said only minor radioactivity was detected at the site. Though the military refused to say, it’s thought the bomb had an explosive yield of 10 megatons, or about 625 times that of the Hiroshima bomb.
Source:
Stephens, John Richard. “Official Documents.” Weird History 101: Tales of Intrigue, Mayhem, and Outrageous Behavior. New York: Barnes & Noble, 2006. 39. Print.
Further Reading:
Thermonuclear weapon / Hydrogen bomb / H-Bomb
I'm no expert, but by conventional I believe they always mean non-nuclear. Good point about the smaller atomic bomb, though. That's worth Googling after class.