6. Two nuclear weapons cores vanished over the Mediterranean Sea
On March 10, 1956, a USAF B-47 Stratojet embarked on a classified mission from MacDill Air Force Base, Florida, to an unspecified overseas base. It’s objective was the safe and secure couriering of two nuclear weapons cores for an unknown purpose. In order to minimize risk to the cargo or crew from either landing malfunctions or potential hijacking the mission was non-stop, and accordingly flying thousands of miles without landing necessitated multiple mid-air refueling rendezvouses. The first of these meetings occurred without any noteworthy concerns, with the aircraft successfully detaching from the tanker and continuing along its given trajectory.
The second refueling location was over the Mediterranean Sea, and required the aircraft to decrease its altitude to 14,000 feet. On March 10 there was an especially solid cloud formation over the Mediterranean, the base of which formed at 14,500 feet and into which the B-47 descended. After making this descent into the clouds the B-47 and its crew were never heard from again, failing to rendezvous with the second refueling plane or transmit any further communications. Despite a considerable search effort by multiple nations, no trace of the aircraft, the crew, or the nuclear weapons cores have ever been found, and it is unknown whether the cores reside at the bottom of the Mediterranean Sea or elsewhere.