For decades, people have reported seeing strange objects and unexplained lights in the sky. While many sightings eventually have ordinary explanations, a small number remain difficult to explain, which has helped keep one of the world's biggest conspiracy theories alive: have governments discovered evidence of extraterrestrial life and kept it secret?
The subject has become particularly interesting in recent years because UFOs are no longer discussed solely by conspiracy theorists. The term UAP, meaning Unidentified Anomalous Phenomena, is now commonly used by governments and researchers when discussing sightings that haven't yet been identified.
Some former military personnel and government officials have made extraordinary claims about secret programmes supposedly involving recovered objects or materials. Those claims have attracted enormous attention, but extraordinary claims require strong evidence, and publicly available evidence has not established that governments possess alien spacecraft or extraterrestrial beings.
Then there are the famous cases that have kept the subject alive for generations. Roswell is probably the best known. Something crashed near Roswell, New Mexico, in 1947, and although the US military later explained the debris as being associated with a secret surveillance balloon programme, theories about a recovered alien spacecraft have persisted ever since.
Area 51 has similarly become almost inseparable from UFO folklore. The highly secretive military facility in Nevada really has been used to develop and test classified aircraft, which provides a rather less exciting explanation for at least some unusual sightings in the area. Nevertheless, the secrecy surrounding the site helped create decades of speculation about what might be hidden there.
There is also a huge difference between believing extraterrestrial life exists somewhere in the universe and believing extraterrestrials have visited Earth. Considering the enormous number of stars and planets that exist, many people think some form of life elsewhere is plausible. Demonstrating that such life has actually travelled here is an entirely different matter.
Perhaps that's what makes this conspiracy theory so enduring. Unlike some theories that can be conclusively disproved, the possibility of life elsewhere remains an enormous unanswered scientific question.
So where do you stand? Do you think Earth has ever been visited by extraterrestrial life, do you think governments know more about UFOs than they publicly admit, or do you think there's a perfectly ordinary explanation behind virtually every sighting?
The subject has become particularly interesting in recent years because UFOs are no longer discussed solely by conspiracy theorists. The term UAP, meaning Unidentified Anomalous Phenomena, is now commonly used by governments and researchers when discussing sightings that haven't yet been identified.
Some former military personnel and government officials have made extraordinary claims about secret programmes supposedly involving recovered objects or materials. Those claims have attracted enormous attention, but extraordinary claims require strong evidence, and publicly available evidence has not established that governments possess alien spacecraft or extraterrestrial beings.
Then there are the famous cases that have kept the subject alive for generations. Roswell is probably the best known. Something crashed near Roswell, New Mexico, in 1947, and although the US military later explained the debris as being associated with a secret surveillance balloon programme, theories about a recovered alien spacecraft have persisted ever since.
Area 51 has similarly become almost inseparable from UFO folklore. The highly secretive military facility in Nevada really has been used to develop and test classified aircraft, which provides a rather less exciting explanation for at least some unusual sightings in the area. Nevertheless, the secrecy surrounding the site helped create decades of speculation about what might be hidden there.
There is also a huge difference between believing extraterrestrial life exists somewhere in the universe and believing extraterrestrials have visited Earth. Considering the enormous number of stars and planets that exist, many people think some form of life elsewhere is plausible. Demonstrating that such life has actually travelled here is an entirely different matter.
Perhaps that's what makes this conspiracy theory so enduring. Unlike some theories that can be conclusively disproved, the possibility of life elsewhere remains an enormous unanswered scientific question.
So where do you stand? Do you think Earth has ever been visited by extraterrestrial life, do you think governments know more about UFOs than they publicly admit, or do you think there's a perfectly ordinary explanation behind virtually every sighting?