In its purest form, the UFO has been with us at least as long as man has been capable of making records. Cave paintings and ancient carvings show animals "leaping through the heavens". Unexplained lights and objects in the sky have been written about for thousands of years, usually in religious texts.
The ancient Hindu epic, the "Ramayana",tells of four different types
of flying craft known as "Vimanas" described as two-deck circular craft
with viewing "portholes" and a "dome". They flew with the "speed of the
wind" and gave forth a "melodious sound". Another contemporary source translates
to the following description: "Bhima flew along in his car, resplendent
as the sun and loud as thunder... The flying chariot shone like a flame
in the night sky of summer ... it swept by like a comet... It was as if
two suns were shining. Then the chariot rose up and all the heaven brightened."
Another famous example is that of the book of Ezikiel (Ez. 1:4 - Ez. 1:28) in the Judao-Christian Old Testament, describing the descent of a huge ornate "wheel" from the sky, stirring up a storm in its wake. From the New Testament, consider also the "Star of Bethlehem" (Matthew 2) which moved through the skies to guide the three wise men. Each generation has interpreted these phenomena in light of its own experience and preconceptions.
During 1896 there was a spate of sightings of "strange craft" in the skies above San Fransisco, California and Cherokee, Butte County U.S.A. - these craft had no wings or propellors, but maneuvered vertically as easily as horizontally and emitted a "great light". Craft of the same description were sighted over Oakland, Ca. on November 23rd of that year, and were assumed to be an "implement of aggression" related to the ongoing Spanish-American War! (For further info. on American incidents in this period, see "The Great Texas Airship Mystery." by Wally Chariton)
In the United Kingdom between 1909 and 1913 there was a spate of sightings of "phantom airships", believed to be the new German Zeppelin aircraft spying on the country prior to an invasion attempt. This belief was widely held despite the fact that before 1911 the Zeppelins were too unreliable to undertake such a long flight, and none of the people who "positively identified" these craft as such had ever seen a Zeppelin! Physical evidence of their passing was provided in the form of "hot coals" falling from the skies, lighting up the night and burning patches of grass in ways strikingly similar to modern reports of UFO phenomena. Contemporary reports even talk of the machines landing to steal cattle!
During the latter part of World War Two, Allied planes filed numerous reports of "Foo Fighters" - balls of blinding orange light which travelled at speeds in excess of 500MPH, performed "impossible" manoeuvers, were generally invisible to RADAR and appeared to be under "intelligent control". Recently published documents suggest that these may have been a Nazi secret weapon, remote controlled and intended to destroy entire bomber squadrons before it could be intercepted. However, the technology described is still beyond our grasp (as far as the public are aware...), and so is unlikely to have been available to the Nazis at that time. (One other flaw is that these machines were said to generate extreme broadband electrical signals to "jam" RADAR - so how did their "remote control" systems get through this?)
In the late 1940s, when the term "Flying Saucer" was coined, the "Cold War" was upon us, and "Flying Discs" were assumed to be some form of secret Soviet spying device - by the late 1950s, the world was obsessed with space - Sputnik 1 had just been launched, and all eyes were turned skyward. Little wonder, then, that this generation interpreted any unexplained lights in the skies as "obviously" being craft from space!
Around this time, there arose a popular connection between these "craft" and another phenomena of "abduction" by small, grey creatures and missing time. Even this was not new, as from medieval times there had been stories of people (mainly children) being abducted by faeries and goblins, then taken inside "faerie mounds", only to be released some considerable time later to find that their parents and friends had aged, while they had not... (for "faerie mound" read "parked saucer", for "goblin" read "grey"...) These stories proliferated throughout Europe, with the same details emerging throughout a wide diversity of cultures (sounds familiar?). These "abduction" phenomena seem to exist in most times and cultures, but their connection with UFOs can be described as merely circumstantial, in that they sometimes, but not always, occur together.
By the 1980s, the world was becoming obsessed with the environment. At about this time, explanations for UFO activity began to hinge on such phenomena as aggregations of luminous insects, "ball lightning", gas bubbles from beneath the ocean and "stresses in the Earth's crust"... Again, the explainations fitted in with the prevailing belief system of the time.
It seems obvious that whether or not any of the many explainations for these phenomena are true, there is something happening in the skies. Even the most hardened sceptic cannot explain away all the reported sightings, but many still claim that if you can explain 95% of them, then the other 5% must be explainable or deliberate hoaxes! I believe that there probably is a "rational" explaination for these phenomena, but that we haven't discovered it yet.