Military crafts, changes as new technology is available. I do not believe in UFO's so that's just my opinion.
The Kenneth Arnold Sighting - first UFO report Introduction On June 24, 1947, Kenneth Arnold saw something that would not only change his life forever, but his experience also served as a catalyst for our modern UFO craze. His report after seeing nine unidentified flying objects, is generally considered to be the first widely reported UFO sighting in the United States. Kenneth Arnold Sighting Arnold, a recreational private pilot, was returning home from a flight in Wyoming when he received a radio signal requesting assistance in the Yakima, Washington area. It seems there was a missing troop transport and a aerial search party was being assembled. At 3:00 p.m., flying at 9,000 feet, a bright flash of light caught Arnold's attention. Turning to look our of the side window of his airplane, Arnold was amazed to see 9 saucer shaped objects flying in formation. Thinking they were some sort of military aircraft, he watched them intently as they bobbed, weaved, and darted about flying at an amazingly high rate of speed. What fascinated Arnold the most was the fact that the flying objects had no tail, but rather were round, saucer shaped, metallic and highly polished.
Originally posted by Rubicant13
This one is easy. The aliens get tired of their out-dated model ufo's and upgrade to the newer nicer ones. Nothing is better than that new ufo scent.