Stone circles are also intriguing. There's literally thousands of them that are known about, many others probably dismantled or built on centuries ago,
Made by pre-historic people who supposedly were mainly concerned with food and shelter, but who also had the time and the energy to drag 20 ton boulders about the landscape, shape them, and stand them up in circles.
Perhaps it was just superstition / ritual / religion, but that explanation has never sat comfortably with me. Was there some practical benefit unknown to us? It seems like a lot of effort to expend in a world where if you don't have enough food in store, you won't survive the winter.
Likewise the Giza pyramids where we are expected to believe that they were built by a primitive agrarian society that had not yet invented the wheel.
If you look at the engineering in those buildings, that's clearly a ludicrous assertion.
Not saying it was aliens, but there's no question that there's a piece missing from the jigsaw.