A lot of speculation has been circulating about the existing of current spacecraft using anti-gravity technology. Such as Tr3b Black Manta secret US Air Force plane. This is a triangular shape often taken as UFO's.
Propulsion would come from highly pressurized mercury accelerated by nuclear energy. A plasma is produced which creates an anti-gravity field. With an electromagnetic coil at its center, it intearcts with the field at the quantum level and the thrust is coming from high energy plasma. This spacecraft is undetectable by radar and is capable of making high-g turns and strong velocities.
As Dr. Salvatore Pais (inventor of such tchnology) mentioned "In other words , it effect of cohering the highly energetic and random quantum is possible to reduce a craft ' s inertia , that is , its resistance to vacuum fields ' fluctuations , which virtually block the path motion / acceleration by polarizing the vacuum in the close of an accelerating craft , in such a manner that the resulting proximity of the moving craft . As a result , extreme speeds can be achieved.
https://www.youtube.com/watch?v=2KHVJiUwoYI
see also https://en.wikipedia.org/wiki/Salvatore_Pais
I found that very fascinating. What about you ?
Maybe that explains a lot of UFOs observations.