Black Helicopter and UFO's over Lehigh Acres, Florida USA

An unidentified flying object (usually abbreviated to UFO or U.F.O.) is any unusual apparent object in the sky whose cause cannot be identified by the observer, or (in a narrower definition) by investigators; though in popular usage it more loosely means alien spacecraft, being one explanation (among several) offered for such sightings. Though UFO sightings have occurred throughout history, modern interest in them dates from World War II, since when governments have investigated UFO reports, often from a military perspective, and UFO researchers have investigated, written about and created organizations devoted to the subject.

UFOs have been subject to investigations over the years that vary widely in scope and scientific rigor. Governments or independent academics in the United States, Canada, the United Kingdom, Japan, Peru, France, Belgium, Sweden, Brazil, Chile, Uruguay, Mexico, Spain, and the Soviet Union are known to have investigated UFO reports at various times.

Following the large U.S. surge in sightings in June and early July 1947, on July 9, 1947, Army Air Force (AAF) intelligence, in cooperation with the FBI, began a formal investigation into selected best sightings with characteristics that could not be immediately rationalized, which included Kenneth Arnold’s and that of the United Airlines crew. The AAF used "all of its top scientists" to determine whether or not "such a phenomenon could, in fact, occur". The research was "being conducted with the thought that the flying objects might be a celestial phenomenon," or that "they might be a foreign body mechanically devised and controlled." Three weeks later in a preliminary defense estimate, the air force investigation decided that, "This ‘flying saucer’ situation is not all imaginary or seeing too much in some natural phenomenon. Something is really flying around."

A further review by the intelligence and technical divisions of the Air Materiel Command at Wright Field reached the same conclusion, that "the phenomenon is something real and not visionary or fictitious," that there were objects in the shape of a disc, metallic in appearance, and as big as man-made aircraft. They were characterized by "extreme rates of climb [and] maneuverability," general lack of noise, absence of trail, occasional formation flying, and "evasive" behavior "when sighted or contacted by friendly aircraft and radar," suggesting a controlled craft. It was thus recommended in late September 1947 that an official Air Force investigation be set up to investigate the phenomenon. It was also recommended that other government agencies should assist in the investigation.