I have also heard that the most ufo's were seen in the nevada region, brazil, mexico for number of times. They were also appeared in the sphinx area of egypt, they were also observed at the top of the vulcanic erruption area.
Hydrogen is their fuel?
what is the reality?, are all ufo's are techonologically more advance than of earth's?
Why they appear with spped and went away with great speed?
Are they mostly caught on the radars previously?
What standards are derived for its realisation, as they were located in the sky? or suppose 1 or 2 big tail lights of the fighter plane looks like a ufo?