kdawson writes "io9 has a scary outline of five times the US came close to accidental nuclear disasters. Quoting: 'In August of 1950, ten B-29 Superfortress bombers took off from what was then called Fairfield-Suisun Air Force Base in California, headed for Guam. Each was carrying a Mark IV atom bomb, which was about twice as powerful as the bombs dropped on Japan at the end of World War II. Shortly after takeoff, one of the B-29s had engine trouble. On board was General Robert Travis. He commanded the plane to turn back to the base when the landing gear refused to retract. Sensing the plane was going down, the pilot tried to avoid some base housing before crashing at the northwest corner of the base. The initial impact killed 12 of the 20 people aboard, including General Travis. The resulting fire eventually detonated the 5,000 pounds of conventional explosives that were part of the Mark IV. That massive explosion killed seven people on the ground. Had the bomb been armed with its fissile capsule, the immediate death toll may have reached six figures.'"