Some folks (Lue Elizondo for one) have suggested that we are seeing something that is moving between two dimensions.
Not sure how current physics would account for such, but there are observations that would fit nicely into that scenario (like beings that appear to walk through walls or windows).
An Air force major, a B47 pilot, whom I knew very well, told me he saw a UFO silver shiny saucer in a vertical dive, buzz a B47 flying north in 1957 at low altitude, execute a 90 degree turn over the tail, fly from tail to over the canopy, and exit at 3000+ mph due north at 500 feet off the ground. He said the saucer was flying at a ~30 degree angle, not level, when it exited the area of view. He had binoculars on it all the way. He said it never slowed down from the dive.