On June 24, 1947, Kenneth Arnold, a businessman and air rescue pilot, was flying his small plane over the Cascade Mountains in the state of Washington. On the lookout for a crashed airplane, he instead witnessed what he said were nine bright glistening objects flying at speeds beyond the capabilities of any known aircraft at the time. Arnold later told a reporter that the entities "flew erratic, like a saucer if you skip it across that water." Accounts of his encounter with "flying saucers" quickly spread through the news wires. Over the next several years thousands of other people reported seeing flying saucers in the skies.

The flying saucer phenomenon sparked widespread attention not only from the media and the public, but also from the U.S. government. Between 1948 and 1969, in what eventually became known as Project Blue Book, the U.S. Air Force maintained a permanent team of investigators...