If they were spacecraft, the design would have to be at least centuries ahead of anything we have, based on completely unknown scientific principles, in order to have the extremely advanced abilities reported, such as exhibiting accelerations many times more severe than any human could endure, or the ability to suddenly disappear into thin air.  It would seem strange that such advanced craft would explode on a regular basis, be subject to multiple crashes, or descend with a " falling leaf " motion.
 
 Although many people have claimed to be abducted by the alleged aliens, there does not appear to be any physical evidence whatsoever of such an activity.  Abductees have often reported having small metal implants inserted, none of which have ever been found.  In spite of multiple communications, no new scientific information has been reported, nor have any of the alien predictions come true. Women have often reported being made pregnant by aliens, even in cases of infertile females, but no hybrid fetuses or pregnancies have been found.

  The popular press often presents the majority of the aliens as the " grays ", a form basically unheard of until the early sixties.  Most earlier encounters described tall blondes, often from Mars or Venus.  The Grays did not actually become popular until after such creatures had been portrayed in several science - fiction movies, complete with abductions, big eyes, loss of memory, and the use of mental telepathy.  They had also become widely known in the popular press, including drawings.

 There have been other changes in the popular mythology.  In just the last last few years, abductions have been reported without any sighting of spacecraft, with the aliens simply passing through solid walls.  Some researchers feel the alleged abduction may consist of a dream state, in which the subjects feel they are awake, when they actually are not, and may have even been sleepwalking at the time.

 Although the descriptions of aliens have been considered by some enthusiasts as being consistent, they usually are not.  Fingers are described anywhere from three to six.  Although the popular press often describes them as being consistently gray, they are also reported as white, black, tan, green, etc.  Some modern forms were described as having crab claws or feathers.  One researcher found the aliens to be consistently sinister, while another felt they were almost always kind and benevolent.