In January 1948, Capt. Thomas Mantell, a Kentucky Air National Guard pilot, and some other pilots took off in P-51 Mustang fighters from the airfield at Fort Knox, pursuing a rapidly moving, westbound, circular object. The other pilots called off the chase when the large, metallic object elevated to 22,500 feet (6,858 meters) in altitude, and they returned safely. (The danger zone for flying without oxygen begins at 14,000 feet, or 4,267 meters.) But Mantell kept following this strange aircraft. His plane tumbled from the sky and crashed in Kentucky.
The official explanation was that he lost consciousness from a lack of oxygen, but questions remain about why he kept going and what he was following so intently. Air Force officials initially said Mantell had mistakenly followed planet Venus, but some witnesses thought he had seen a spaceship or some other UFO. The object is now believed to be a Skyhook weather balloon used to measure radiation levels that was part of a secret project [sources: National Guard History eMuseum, Randle].