One of the most complicated topics of all time. I'm relatively certain of is that some of the UFO phenomenon is man-made, most likely classified military technology. Gordon White
made a great comparison a few years back... what would the average person in the mid-1960s, think of the SR-71
, which wasn't declassified until the early 90s. It would truly appear "out of this world."
This is confounded through by what appears to be another, seemingly longer lived intelligence at play. There's no shortage of odd sightings recorded throughout recorded history, and as Jacques Vallée noted, they seem to take the appearance of the dominant religious paradigm of the time. The Fatima miracle
for example. There seems to be an element of perception here... that in a sense we are projecting the form to match our belief structure, that's why we see miraculous machines flying through the sky (if one sees anything solidly definable at all). This is aided if you consider alien abduction stories (or encounters with mythological creatures/strange people/etc historically) as part of the UFO issue, which I think is more or less reasonable. Reports of odd phenomena in the sky often do precede the "close encounter."
As for what causes these events, it could be physical beings that are more "advanced" than us (our creators?), beings that exist on different planes of reality, members of a human secret society that's spanned generations, and/or the Creator of all existence. I personally lean towards the "inorganic beings screwing with us" but nothing about this is clear cut. Except that if the media and government is telling you it's little green men from outer space, that's likely wrong.
Just watch the first 30 seconds where the MP is talking:
Note to any children out there, if someone ever says "frightfully excited" in any context, get away as quickly as you can and find an adult.