UFOs, and what they are, have been a subject of debate throughout the world for decades now.
For those who are apart of the UFO community, UFOs are usually considered to be extra-terrestrial spacecraft. For skeptics however most UFOs can be easily explained as either being some type of natural weather or astronomical phenomenon, a mis-identification of a man made object, or a hoax.
Beside the most obvious explanations for what UFOs are, there are a few not so obvious (or in some cases, accepted) explanations for what UFOs really are:
Probably one of the far most common alternative UFO explanations is that UFOs (and the alleged beings operating them) are actually from other universes, rather than other planets.
This explanation has become so common among UFO believers that many believers actually try to determine if a UFO in a photo or video is of either extra-terrestrial or extra-dimensional origin (rather than of natural or man made origin).
7. Time travelers
Another explanation for the origin of UFOs is that they are from Earth, just far, far into the future. Just how far exactly is often debated amongst the UFO believers who believe this theory.
Some people believe that they're from a few hundred years into the future. Some people believe that they're from several millions of years into the future. Some people even believe that they're just from a few decades into the future.
6. Angels and Demons
Probably one of the more popular explanations amongst devout and fundamentalist Christians (especially those who don't believe in even the possibility of extra-terrestrial life existing elsewhere in the universe) is that UFOs are either angels sent by God, or (more commonly) demons sent by Satan.
While this explanation is far more accepted with devout and fundamentalist Christians (at least those who actually believe in UFOs to begin with) most UFO believers (and skeptics for that matter) do not.
5. Secret and experimental military aircraft
One of the more common explanations for UFOs amongst both skeptics and UFO believers is that many UFOs are actually secret and experimental military aircraft.
The reason why this is accepted among skeptics as well as UFO believers is because several different kinds military aircraft, back when they were still secret and/or experimental, were thought to be UFOs (i.e. F-117, B-2) and that sometimes the government even used the UFO phenomenon as a cover to help keep certain aircraft secret for years (like with the SR-71).
While skeptics and UFO believers do accept that some UFOs are most likely secret and experimental aircraft, many UFO believers also believe that these aircraft are far more technologically advance than what anyone believes is currently possible, and that the technology being used in them is of extra-terrestrial origin.
Some UFO believers believe that some UFOs aren't even physically real at all, and that they're actually holograms created by the military.
While the main reasons behind why the military would even do this in civilians areas (if they even are doing this at all) are unknown, speculations from UFO believers vary from either being tests for holographic projection weapons, to being part of a bizarre conspiracy theory called Project Blue Beam.
Particularly popular amongst supporters of the hollow Earth theory, some people believe that UFOs are actually created by beings that supposedly live inside the Earth.
This theory is of course impossible, being that modern science has completely dis-proven the hollow Earth theory, although some will say that there are large areas underground that are hollowed out, not the whole Earth, and that these alleged beings live in those areas (at least those that don't claim that there is a conspiracy to hide the "truth" about the Earth being hollow).
Some people in the UFO community believe that UFOs are neither of alien, or even of technological origin (or natural either) and that they're actually supernatural in nature.
While the belief that UFOs are actually spirits is somewhat popular amongst those in the New Age Movement, the explanation that UFOs are spirits are usually dismissed by both skeptics and UFO believers alike.
1. Ancient human astronauts
Some people believe (especially those who believe in the myth about Atlantis) that humans were more technologically advanced in the past than we were actually known to be, and that some even managed to build spacecraft and left the Earth thousands of years ago, and that they sometimes come back to Earth from time to time.
While this theory may sound plausible, so far no evidence has ever been found that we are far more advanced technologically in the past then we are today.