You can come up with any number of scifi-ish explanations: aliens, humans from the future, lost/forgotten civilizations. But it could also be someone testing an uncommon craft.
Frankly, we don't know, and one of the reasons is, I believe, people are hesitant to take a serious look at it (or even talk about it) because they don't want to be classified as kooks.
Frankly, we don't know, and one of the reasons is, I believe, people are hesitant to take a serious look at it (or even talk about it) because they don't want to be classified as kooks.