The technology of flight on Earth has been use of air pressure (for lift) or rocket propulsion. A circular craft such as those registered in the 40s and 50s by our radar and observations by our pilots is not using either of those technologies.
Do you have a top secret clearance for Area 51 and the top secret facility in Ontario? If not, then how would you know? I believe that what I saw about the technology means they are trying to perfect it, so our military can use it, but it has issues.
People choose what they believe, and I do not believe in little green men, or Aliens from outer space, sorry.