Aliens and UFOs have been such a staple of American pop culture it's hard to believe the fascination only began about 50 years ago. It started with a spark of interest, exploded and continues to this day in a constant burn.
The UFO era began on June 24, 1947, with a recreational pilot named Kenneth Arnold, says Jerome Clark, author of "The UFO Encyclopedia."
Clark describes Arnold as a guy with not a lot of imagination. He didn't read science fiction. He didn't have any occult interests. "He lived pretty close to the ground, mentally," Clark told ABC News.
So, Clark says, when Arnold saw nine disc-shaped objects flying at some considerable speed over Mount Rainier in Washington, the pilot thought that he was witnessing a secret military experiment.
Arnold died in 1984, but in an interview at the time, he said the objects "looked something like a pie plate that was cut in half with a sort of a convex triangle in the rear. I'd be glad to confirm it with my hands on a Bible, because I did see it."
The press called the nine objects Arnold described "flying saucers." Because Arnold was regarded as a credible witness, Clark said the press didn't believe he was making up the story.
In the next two or three weeks there were many hundreds of sightings, Clark said.
In response, the Army Air Force (the precursor to the U.S. Air Force) launched an investigation, and on Sept. 23, 1947, Gen. Nathan Twining categorically stated "the phenomenon reported is something real and not visionary or fictitious."
It was the beginning of the Cold War and Americans feared a national security problem -- that airspace was being penetrated by the Soviet Union. The government established a full-time office to investigate the sightings, and quickly came to realize that they were not dealing with Soviet aircraft. Nothing man-made performed like the reported flying saucers.
Then two experienced commercial pilots, Clarence Chiles and John Whitted, reported their own encounter with a UFO immediately after it happened over Alabama on July 24, 1948.
They said it was a cigar-shaped object, perhaps 100 feet long, flying faster than any aircraft they had ever seen. It had two rows of windows and they could see lights inside and orange-red flames shooting out of the tail.
Air Force investigators trusted the testimony of these experienced pilots. More pilot accounts emerged.
In the fall of 1948, Air Force investigators sent a top-secret report to Gen. Hoyt Vandenberg, the Air Force's chief of staff, saying Earth was being visited by alien spaceships.
Vandenberg remained unconvinced. The Air Force wanted the flying saucer phenomenon to go away, but the popular fascination with mysterious flying objects was growing.
In 1952, the Air Force received more UFO reports than any other year in history. "Most alarmingly, the sightings were focused on Washington D.C.," Clark said. "At least three radar systems in Washington, in the area, were picking up unidentified targets." It looked like the most important airspace in the country was being penetrated.
An Air Force Air Intelligence Report in December 1948 said there was a public hysteria.