Do you think UFOs might be human-made craft that the US government is testing? Like Aurora?
UFO's are an "All of the above" proposition. They are many things. The "original" sighting of a "flying saucer" was clearly a sighting of a Northrop Flying Wing aircraft. It was described properly, the term "saucer" came from the description of how it flew, not it's shape. Disc shaped aircraft have been around a long time. Plenty of pictures and evidence can be found on the web. The "flying pancake" certainly comes to mind.
I have in mind my own design for a "flying saucer". It works with jet propulsion. No anti-gravity, alien technology or any of that.
How it works:
An airplane moves through the air. Faster moving air above the wing causes low pressure, higher pressure below provides lift.
A helicopter moves rotors through the air. Same principle as an airplane except the machine stays stationary while the rotors move.
My "flying saucer" design works by moving the air itself. Jet engines are used to circulate the air above the disc. This caused the air above it to become low pressure and lift is created. I'll have to post images for the design at a later date. (Actually, I have two different designs.)
This principle is used on some STOL aircraft designs in the '70s. There was one STOL cargo plane design I remember years ago that placed the engines above the wings to use the same principle. This gave it the ability to land and take off in very short distances. This principle is how tornados and hurricanes can lift the roof off of a house.
Here is another workable flying saucer design that could actually work. It really relies on vectored thrust, like the Harrier jump jet, only disc shaped.
Silver Bug
Another web site on this particular design:
Silver Bug 2
There are other designs as well, all work without anything science doesn't already know.