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Arnold was tryging to find out what flying saucers were.
The notion that they came from Mars was already there. Arnold himself told that days after his sighting was in the newspapers, a woman pointed at him in a cafe and shouted excitedly "there's the man who saw the men from Mars!". Occasional newspaper articles devised about possible Martian visitors, had had Charles Fort decades ago.
But Arnold, maybe for fear of being called a crackpot, or sincerely, pursued other tracks. He wondered, for example, whether the saucers were some sort of unknown flying animal, or if they were top secret Army devces.
He collected weird news, just like Charles Fort had.
Some "skeptics" have claimed that Arnold was a crackpot because he had spoken of "unidentified submarines". But such sightings were indeed reported. Reports of that would be called USO, for "Unknown Submarine Objects", later, appeared in the newspapers. He did not make these up, he just collected anything that may have directed him to a solution of the flying saucer mystery.