In fact, U.S. government agencies often go to great lengths to secure lost weapons. One such incident occurred on Jan. 17, 1966, when a B-52 and a KC-135 refueling tanker collided over southern Spain and scattered four B-28 thermonuclear bombs around the fishing village of Palomares. The conventional explosives for two of the bombs exploded, but the nuclear components did not detonate because they were not armed. The U.S. military sent troops to pick up the undetonated one that fell on land, clean up the radioactive pieces scattered by the two which detonated, and find the fourth which landed in the sea. The U.S. government even dispatched a submarine to find the one in the Mediterranean Sea. Called ‘Alvin,’ the small deep-ocean sub was high-tech for its time, but the crew nearly died when the sub was almost entangled in the parachute that was still attached to the bomb on the ocean floor. Meanwhile, the service members who helped find the landward bombs and clean up the wreckage also developed cancers which they say are linked to that mission 56 years ago.

Considering the extent to which the U.S. looks for lost nukes like it did in Palomares, the stories behind the five instances where recovery crews could not locate or recover weapons are extraordinary. Below is a list of those five accidents, one of which resulted in two missing nuclear weapons. Keep in mind that in all but one, the lost nuclear weapons did not include the pit or capsule that contains the components for triggering a nuclear detonation. That means we can all sleep a little easier knowing those weapons cannot blow up a city. However, the U.S. government still classifies those pit-less devices as nuclear weapons: sophisticated, expensive machines that at the time were closely-guarded tools of mass destruction. And there are many more out there from other governments like the Soviet Union which may never be found.