At about quarter past six on the evening of December 6, 1944, three men and a woman working outside a coal mine near the town of Thermopolis, Wyoming, heard a soft whistling sound overhead. There was an explosion. They looked up and saw flames high in the sky, and something like a parachute floating down. The next morning, searchers found a small crater nearby.
Less than a week later, two men stumbled on the crumpled remains of what seemed to be a large balloon strewn in a forest near Kalispell, Montana. They reported the incident to local authorities, who in turn notified the FBI. Inspection of the debris revealed Japanese markings. The Army, Air Force and Navy were informed of the discovery.