After WW2, we took plans from the Germans to make aircraft. Many of these became experiments that were tested away from prying eyes. There were a lot of experiments with radar-avoiding craft and supersonic designs.
All crashes were heavily guarded and kept secret. Who knows why they are still secret today.
Some people claim the Nazi's had a UFO and were busy trying to reverse engineer it.
I've been rather skeptical. But there's something in that direction about the Nazi's.