The general view is that human race only exists as an extreme coincidence and we wont be staying around for much longer, meaning that we will only have been around for an extremely short time in the great scheme of things. So the chances of another form of intelligent life coincidentally coming in to existence at around the same time and is still around is extremely low, then adding on top of that the probability of the these "aliens" building technology that would be able to find us and them being able to get to us during this short time is so low that it is bordering upon impossible. Meaning that the chances of these ufo's originating from another planet is neigh on impossible, the most probable explanation is that these objects are actually governmental secrets of some global superpower pissing about trying to get the upper hand on another country. This is one of the theories which is totally speculative,Aurora Another theory is the americans trying to create machines to alter the effects of gravity enabling extremely quick flight and maneuverability, and because the only way to do this is to have the craft in the shape of a saucer can mean that nearly all sightings could possibly be related to it.Avro