UFO or unknown aircraft?

So many reports of UFOs - are all these reports relating to currently unknown aircraft or are they alien visitors? Jury's OUT!
College physics teacher describes close encounter with silent triangle UFO
A California college physics teacher watched a triangle-shaped UFO that came within several hundred feet and then silently moved away at 9 p.m. on April 15, 2010, in Ventura, according to testimony from the Mutual UFO Network (MUFON) database.

The instructor first heard a screech sound, then watched the object move quickly across the sky.

"At first it looked like it was a large bird high in the air as the front and two sides were heavily darkened against the mostly dark sky," the witness stated. "It was moving toward me and over a period of about 2 seconds it became clearer and resembled a hang-glider. It was descending and moving toward the east and seemed to be gliding smoothly, making no sound that I could hear."
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