type="html">Evidence of UFO encounters through mankind's history -- from prehistoric cave paintings, to Medieval frescoes, to Renaissance art that depict strange objects in the sky.
While technically a UFO refers to any unidentified flying object, in modern popular culture the term UFO has generally become synonymous with alien spacecraft;however, the term ETV (ExtraTerrestrial Vehicle) is sometimes used to separate this explanation of UFOs from totally earthbound explanations.
Proponents argue that because these objects appear to be technological and not natural phenomena and are alleged to display flight characteristics or have shapes seemingly unknown to conventional technology, the conclusion is that they must not be from Earth. Though UFO sightings have occurred throughout recorded history, modern interest in them dates from World War II (see foo fighter), further fueled in the late 1940s by Kenneth Arnold's report of a close encounter, which led to coining of the term flying saucer, and the Roswell UFO incident. Since then governments have investigated UFO reports, often from a military perspective, and UFO researchers have investigated, written about, and created organizations devoted to the subject. One such investigation, the UK's Project Condign, made public in 2006, attributed unaccountable UFO sightings to a hitherto unknown and scientifically unexplained "plasma field." It also concluded that Russian, former Soviet Republics, and Chinese authorities had made a co-ordinated effort to understand the UFO phenomenon and that military organizations, particularly in Russia, had done "considerably more work (than is evident from open sources)" on military applications stemming from their UFO research. The report also noted that "several aircraft have been destroyed and at least four pilots have been killed 'chasing UFOs'."
UFOs are sometimes an element of conspiracy theories in which governments are allegedly intentionally "covering up" the existence of aliens by removing physical evidence of their presence, or even collaborating with extraterrestrial beings. There are many versions of this story; some are exclusive, while others overlap with various other conspiracy theories.
In the U.S., an opinion poll conducted in 1997 suggested that 80% of Americans believed the U.S. government was withholding such information. Various notables have also expressed such views. Some examples are astronauts Gordon Cooper and Edgar Mitchell, Senator Barry Goldwater, Vice Admiral Roscoe H. Hillenkoetter (the first CIA director), Lord Hill-Norton (former British Chief of Defense Staff and NATO head), the 1999 French COMETA study by various French generals and aerospace experts, and Yves Sillard (former director of CNES, new director of French UFO research organization GEIPAN).
It has also been suggested by a few paranormal authors that all or most human technology and culture is based on extraterrestrial contact.