What happened
On June 24, 1947, at around 3:00 p.m., businessman and pilot Kenneth Arnold was flying a small plane from Chehalis to Yakima, while also searching for a missing military C-46. At an altitude of about 2,800 meters (9,200 feet), near Mount Rainier, he saw flashes and nine shimmering objects flying in formation toward Mount Adams. Based on the time it took them to pass between the peaks, he estimated their speed at about 2,700 km/h (1,700 mph) – three times faster than any aircraft. After landing in Pendleton he told reporters about it, saying the objects moved “like a saucer if you skip it across the water.” The press coined the term “flying saucers,” and in the following weeks hundreds of similar sightings were reported across the United States.
Witnesses
Kenneth Arnold; on the same day a prospector in the Cascade Mountains reported similar objects.
Explanations and hypotheses
Skeptics have proposed many explanations: a flock of birds (such as pelicans) seen at a misjudged distance, mirages, reflections off snow, and even fragments of a meteor. Project Blue Book classified the sighting as a mirage, but Arnold firmly defended his account for the rest of his life. Whatever he saw, his sighting launched the modern UFO era.
Disputed A likely explanation exists, but some of the facts are still disputed.
More: Wikipedia (en)