There are more than a few theories as to why UFO's and there passengers have been seen here within the earth's atmosphere.  Reports have been documented for as long as mankind has been recording history.  Below are a few of the most probable theories.  We would very much appreciate your sharing any theories you might have with us.  Just E-Mail and share your theory.  We have organized our theories numerically. 

1.  "THE TOURIST THEORY".  Alien Beings are simply visiting the Earth in their quest for truth much like we are attempting to search the galaxy in our own quest for truth.  Of necessity those actually reaching the Earth would have to be far more technologically advanced than we are.

2.  "TO PREVENT HUMAN SELF DESTRUCTION".  UFO's represent visitors who are in a manner of speaking "sent by God" to intervene in Human affairs since we now have the capability to destroy the planet.