Singular UFO sightings/events
One of the problems with UFO “research” or scrutiny is that significant UFO episodes happen once and are rarely or never repeated, causing absence of the repeatability intrinsic to scientific methodology.
For instance, the World War II phenomenon of “foo fighters” never occurred again, nor the ghost rockets over Scandinavia.
Kenneth Arnold’s chain of “saucers” has not been seen outside the 1947 time-frame.
A Roswell incident has not recurred, Aztec being a misguided extrapolation of the Roswell tale.
The green fireballs over the American southwest in the late 40s and early 50s, despite a spate of later sightings, have not been seen in the sighting numbers that took place originally.
The McMinnville (Trent) object, while duplicated in hoaxes, never appeared again in legitimate photographs.
The odd entities allegedly seen alongside landed aerial craft in the 1950s, mostly in Europe, have not recurred.
While the 1959 Reverend Gill (Papua. New Guinea) sighting provided elements seen in similar sightings (noted here recently), archetypal sightings are missing in UFO reportage.
The giants of Voronezh (Russia), 1989, never showed up again.
The Betty/Barney Hill abduction, the Travis Walton kidnapping, the Pascagoula experience, and Rendlesham kinds of events were one of a kind, not duplicated or even approximated in later or recent UFO reports.
The raft of alien abduction accounts can be placed in a neurological setting, not an actual physical setting.
And yes, UFO sightings over military installations, can be said to have been duplicated and still occurring but no related sighting has the cachet of the sightings noted above.
(There are other singular UFO sightings or episodes that you can, also, cite certainly.)
That such UFO sightings, as those noted, are virtually unique goes to the problem of investigation or scientific scrutiny; one time, singular events lie outside methodologies to explain them.
Why is this so when it comes to UFOs?
That’s the matter “ufology” can’t deal with, and hasn’t.