Project Blue Book episode "The Washington Merry-Go-Round" takes from the unidentified flying objects that flew around the nation's capital.
TR-3B Anti-Gravity Spacecrafts
It doesn't exist officially. It uses highly pressured mercury accelerated by nuclear energy to produce a plasma that creates a field of anti-gravity around the ship. Conventional thrusters located at the tips of the craft allow it to perform all manner of rapid high speed maneuvers along all three axes. Interestingly, the plasma generated also reduces radar signature significantly. So it'll be almost invisible on radar & remain undetected. This literally means that it can go to any country it likes without being detected by air traffic control & air defence systems. Read more at http://en.wikipedia.org/wiki/TR-3_Black_Manta