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.
Threat to National Security
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.
The reports of flying saucers had become an indirect threat to national security, said Robert Goldberg, author of "Enemies Within: The Culture of Conspiracy in Modern America." So many calls were clogging the Pentagon's communications channels, the nation was being left vulnerable to a Soviet attack, he said.
President Truman ordered the CIA to make recommendations on how the Air Force should handle the UFO problem.
The CIA convened a panel headed by the physicist H.P. Robertson. The panel said it was important to remove the aura of mystery from the UFO phenomenon, so that people would no longer take them seriously and would stop reporting them.
Flying saucers were to be made the subject of ridicule. But it proved impossible to control the public interest. One of the most respected publications in America, Life magazine, published photographs of a UFO taken by a farmer in its June 26, 1950, issue.
Hollywood further inflamed the public appetite with movies like "The Day the Earth Stood Still" (1951) and "Earth Versus the Flying Saucers" (1956).
Interest continues to this day. Recent years have seen an explosion of interest in the form of books, TV shows like "The X-Files" and movies like "Independence Day."
A remake of "The War of the Worlds" -- the H.G. Wells classic that Orson Welles turned into a radio broadcast that terrified U.S. audiences in 1938 -- is even due to hit theaters this year.