January 24, 1961
On January 24, 1961, the wing of a B-52 bomber split apart while on an alert mission above Goldsboro, North Carolina. Onboard were two 24-megaton nuclear bombs. One of these successfully deployed its emergency parachute, while the other fell and crashed to the ground. It's believed the unexploded bomb smashed into farmland around the town, but it has never been recovered. In 2012, North Carolina put up a sign near the supposed crash site to commemorate the incident.
December 5, 1965
An A-4E Skyhawk aircraft loaded with a nuclear weapon rolled off the back off an aircraft carrier, USS Ticonderoga, stationed in the Philippine Sea near Japan. The plane, pilot, and nuclear bomb have never been found.
In 1989, the US eventually admitted their bomb was still laying in the seabed around 128 kilometers (80 miles) from a small Japanese island. Needless to say, the Japanese government and environmental groups were pretty pissed about it.
At some point during the Spring of 1968, the US military lost some kind of nuclear weapon. The Pentagon still keeps information about the incident tightly under wraps. However, some have speculated that the incident refers to the nuclear-powered Scorpion submarine. In May 1968, the attack submarine went missing along with its 99-strong crew in the Atlantic Ocean after being sent on a secret mission to spy on the Soviet navy. This, however, remains conjecture.