On January 7, 1948, Captain Thomas F. Mantell, a seasoned pilot of the Kentucky Air National Guard, embarked on a routine flight that would soon become a legendary event in UFO lore. Mantell was flying his F-51D Mustang with three other pilots when they encountered a mysterious, metallic object hovering in the sky.
The object, large and circular, captured Mantell's curiosity, compelling him to investigate further despite the potential risks. As he ascended to a higher altitude in pursuit of the object, Mantell communicated his observations back to his squadron and ground control, describing the UFO as "metallic" and of "tremendous size."
Pushing his aircraft beyond its limits, Mantell climbed above 20,000 feet without oxygen equipment, a decision that would prove fatal. His last transmission reported the object as being directly above him, but soon after, he ceased all communication. It is believed that hypoxia, caused by the lack of oxygen at high altitude, led to Mantell losing consciousness.
His plane spiraled out of control and crashed on a farm near the Kentucky-Tennessee border. The investigation into the crash considered several theories, but the most likely explanation was that Mantell had been chasing a Skyhook balloon. These high-altitude research balloons, which were classified at the time, matched the description of the UFO.