The U.S. Central Intelligence Agency (CIA) reveals that a majority of the unidentified flying object (UFO) sightings in the 1950s and the 1960s were actually sightings of U2 spy planes.

While a UFO is unidentified, it does not mean it is an alien craft. UFO sightings have been reported by people for decades and from various parts of the world.

The federal agency recently tweeted a list of its top 10 most read "Best of 2014" documents. The most read document was from 1998: "The CIA and the U-2 Program, 1954-1974."

The document was originally tweeted by CIA in July 2014, with details about its U2 spy plane, which the agency claims to be responsible for a major chunk of UFO sightings when the spy plane was actually active.

According to the document, the CIA flew the U2 planes at high altitudes of about 60,000 feet over countries such as the then Soviet Union. When U2 spy planes were active, commercial flights reached altitudes of between 10,000 and 20,000 feet. Military planes flew at 40,000 feet or less above the ground. Many people who may have sighted the U2 believed that humans could not reach the height at which the U2 planes flew and the mysterious objects were deemed UFOs.

The document also revealed that when U2 planes started flying at high altitude of over 60,000 feet, the number of UFO sightings reported also increased.

Many UFO reports were made by airline pilots flying in the early hours of the evening. At this time of day, the sun drops below the horizon of a plane that is flying at an altitude of 20,000 feet.

"If a U2 was airborne in the vicinity of the airliner at the same time, its horizon from an altitude of 60,000 feet was considerably more distant, and, being so high in the sky, its silver wings would catch and reflect the rays of the sun and appear to the airliner pilot, 40,000 feet below, to be fiery objects," reported the document.

The increasing number of UFO sightings led to Operation Blue Book, which investigated the UFO phenomenon. Even though the CIA has clarified that most of the UFO sightings in 1950s and 1960s were those of the U2, there are still some people who doubt the CIA's explanation.

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