Here's the deal; UFOs are said to be completely quiet, no sonic boom, not make any splash when submerging into water, and are EXTREMELY agile, with +50G turns, enough to kill any living animal on Earth, yet the pilots never seem injured. But
how? 
I have a theory. The UFOs are propelled by a sort of gravity control, capable of bending spacetime, and radically shortening distances with nothing but gravity. But, as with any theory, there has to be proof. Here's my Three Theses on how they use gravity control to move unlike anything we know today:
1)
They are completely silent, but how? Simple; since they use a gravitational field around the craft, they pull the surrounding air onto themselves. Put it this way, the Earth moves at hypersonic speed throughout the galaxy, but, it does not make a gigantic sonic boom, because all the air around it is held "stationary", from its frame of view. Same with the UFO. Sound is just a pressure wave through air, so with the air around it stationary, we can't hear it at all.
2)
They can pull a lot of Gs without breaking, or injuring the pilot, but how? Because since it is under its own gravitational field, the gravity of the Earth, and therefore, the effect of G forces, is dampened. This means that pilot and craft survive to fly another day.
3)
When a USO submerges underwater, it does not leave a splash, but how? The first thesis should have covered this, because, it'sgravity field pulls the splash water towards it, effectively "muting" the splash.