Googling "mysterious ancient discoveries" or "unexplained ancient inventions" leads to dozens of sites listing artifacts supposedly so baffling the only possible answer could be aliens, time travel, the paranormal, the Iluminati, or aliens. Wait - did we already mention aliens? Sorry: they're a go-to explanation for anything apparently too sophisticated, weird, or "out-of-place" for "humans figured it out, okay?" to be a satisfying explanation.
It's a disheartening discovery, because once you separate the hoaxes and nonsense from the finds of actual archeological interest, there are still cool mysteries to explore. The list below features mysterious ancient inventions, unexplained ancient discoveries, and some slightly more recent finds still baffling to scientists in the 21st century - but not because they're a sign of alien tech. Give ancient humanity a little more credit, y'know?