After WW2 we brought over n azi scientits in "operation paperclip", and they started nasa and continued other scientific work they were working on. There were many n azi submarines that disappeared and many believe they went to south america and antartica.
Admiral Byrd went to Antarctica with a large fleet in operation "high jump", were defeated and came back with an interesting account. Immediatly after this America and Russia start firing nukes straight up in the air "operation fish bowl" under a larger project called dominic. The word dominic means belonging to God. So, you have operation fish bowl belonging to God, where we send nukes up into the air.
The antarctic treaty is signed, where countries all over the world sign in and in which they agree that access to antartica is limited, and normal people cant just go there and explore.
So, what if besides n azi scientist coming here to continue their work, they went to Antarctica, and started developing anti gravitational aircraft, or in underground military bases around the world, and that is what we are seeing now. Advanced weaponry that is unidentified by the normal person, but nothing from a different planet.