What are UFO's? Who is behind them?

Only considering worthwhile UFO reports such as on this website, that were investigated, here are some hypotheses:

  • Extra Terrestrial Hypothesis (ETH): by far, the most popular theory. It proposes that unidentified flying objects are best explained as being physical spacecraft occupied by extraterrestrial life or non-human aliens, or non occupied alien probes from other planets.

    In 1948, the USAF Project Sign produced a Top Secret Estimate of the Situation, concluding that the ETH was the most likely explanation for the most perplexing unexplained cases. West Germany conducted a study from 1951 to 1954, also concluding that UFOs were extraterrestrial. Some CIA scientists were also seriously considering the ETH. In 1999, the French COMETA report (written primarily by military defense analysts) stated that a "single hypothesis sufficiently takes into account the facts and, for the most part, only calls for present-day science. It is the hypothesis of extraterrestrial visitors." Jean-Jacques Velasco, the head of the official French UFO investigation, wrote in 2005 that 14% of the 5800 cases studied by SEPRA were utterly inexplicable and extraterrestrial in origin.
  • Experiments by CIA or NSA or other secret government organisation(s) in order to study human reactions. While this might explain some UFO sightings and "abduction" stories, it is not plausible in the case of UFO waves such as the Belgian wave of 1989 - 1991, or UFO's tampering with nuclear missiles.
  • Time travelers from the future, on a scientific mission and/or on exotic tourism trips.
  • Interdimensional hypothesis (IDH): an idea advanced by ufologists such as Jacques Vallee, that says UFO's and related events involve visitations from other "realities" or "dimensions" that coexist separately alongside our own. With IDH, it is unnecessary to explain any propulsion method because it says that UFO's are not spacecraft, but rather devices that travel between different realities.
  • A hidden advanced civilisation of humans is behind it all. This would be supported by many descriptions of aliens that are humanoid in appearance and seem able to exist on Earth without difficulty, often lacking space suits.
  • A combination of the above.
  • Real UFO's don't exist: all UFO's are either hoaxes, misinterpretations of common objects or phenomena, optical illusions, hallucinations, or are caused by mass hysteria. Disproving this theory is the vast amount of evidence regarding the UFO phenomenon. And many high ranking government officials, scientists, and military experts believe that UFOs should be taken seriously.