Anti-grav vehicles. Supposedly some stealth craft have dual types of propulsion/flight systems.
[to those prone to rabid remarks re such--No, thanks, I already have a container full of tinfoil hats. Start your own collection.]
Stranger things have happened. . . we shall see.
The technology isn't THAT far out. It's cool stuff, but not beyond comprehension ...and it's all based on foundations of prior discoveries. It's not like this stuff just popped out of the blue for no reason. Discovery was accelerated not by some extraterrestrial beings but by a concept rather new to humankind which blossomed only recently- free speech and the free expression of ideas, freedom of religion, and the protections on private intellectual property- all of which gave an incentive for inventors and companies to share their discoveries by openly publishing them and discussing them, and by selling rights to them. That was a big thing since prior to these concepts coming into practice, guilds worked hard to keep their trade secrets exclusive; scientists and other thinkers could be persecuted as heretics, and designs could be stolen, rather than purchased, and those who had not invented them were able to profit on the scientific work of others by ruining the market for the original discoverer, taking his customers. Reforms in Europe and the birth of America as a country finally got things rolling. The old restrictions fell away and were replaced with a desire for progress- we prospered from it. The US drew the greatest minds and entrepreneurs from all over the world, which in turn feuled even more discoveries. It was only natural that such progress occur exponentially.