Unidentified aerial phenomena (UAPs) are airborne anomalies—or optical phenomena that appear as airborne anomalies—that an observer cannot readily identify.
UAPs gained significant public interest following World War II, potentially influenced by advancements in rocket science.
Some view the observation of UAPs as potential evidence of extraterrestrial life visiting Earth.
The first widely recognized UAP sighting occurred in 1947, when businessman Kenneth Arnold reported seeing a group of fast-moving, crescent-shaped aerial objects while piloting his plane near Mount Rainier in Washington.
The term “flying saucer” originated from a misunderstanding of Arnold’s description, in which he likened the objects’ motion to “saucers skipping on water.” A newspaper misinterpreted this as the objects themselves being saucer-shaped, coining the popularized idea of the flying saucer.
In 1948, the U.S. Air Force initiated Project Sign to investigate the surge of UAP reports.
Early researchers in Project Sign often theorized that UAPs were advanced Soviet aircraft; a minority favored the extraterrestrial hypothesis (ETH).
Project Sign transitioned into Project Grudge in 1949 and later into Project Blue Book in 1952—the longest-running U.S. government study of UAPs, operational until 1969. The program was headquartered at Wright-Patterson Air Force Base in Dayton, Ohio.
During its 17-year tenure, Project Blue Book logged over 12,000 UAP incidents.
These sightings were classified into two categories: those attributable to known phenomena (atmospheric, astronomical, or man-made) and those that remained unexplained.
In 1953, the Central Intelligence Agency (CIA) established the Robertson Panel, led by physicist H.P. Robertson, to review Project Blue Book’s findings.
The Robertson Panel concluded that most UAP sightings were explainable by natural events, posed no security threat to the U.S., and provided no evidence of extraterrestrial life.
Portions of the Robertson Panel’s conclusions were withheld until 1979, fueling conspiracy theories about government cover-ups.
In 1966, the U.S. Air Force launched another inquiry led by physicist Edward U. Condon to analyze 59 enigmatic sightings from Project Blue Book. The results were published in the Scientific Study of Unidentified Flying Objects, or the Condon Report.
The Condon Report found no compelling evidence for extraterrestrial origins, contributing to Project Blue Book’s termination in 1969.
Despite this, a minority of scientists continued to support the ETH. Astronomer J. Allen Hynek, who had worked on Projects Sign, Grudge, and Blue Book, remained a prominent proponent. He established the Center for UFO Studies (CUFOS) in 1973.
Governmental interest in UAPs was rekindled in the 21st century with the Advanced Aviation Threat Identification Program (AATIP), a secretive initiative by the U.S. Department of Defense. Officially, AATIP ran from 2007 to 2012 before funding was redirected.
Canada is among the few nations besides the U.S. to maintain detailed records of UAP sightings. Other nations, including the United Kingdom, Sweden, Denmark, Australia, and Greece, have more limited archives.
In certain countries, such as those in the former Soviet Union and the People’s Republic of China, UAP sightings often stemmed from undisclosed military experiments. These governments occasionally encouraged extraterrestrial explanations to mask the true nature of their activities.
Numerous natural phenomena have been proposed to explain UAP sightings, including mistaken observations of Venus, reflections from windows and glasses, or the halo effect caused by camera lenses, which can give illuminated objects a saucer-like appearance.
Psychologists frequently attribute “abduction” narratives to sleep paralysis, a phenomenon where individuals remain conscious but immobilized during transitions between wakefulness and sleep.