First let me say, the only person on this board more skeptical than me is probably Tom, (but I have him in cynicism).
They have been running a lot of ancient astronaut stuff on the History Channel, hours of it in fact. 99% of it ranges from utterly ridiculous to laughable, but I must admit the remaining 1% is interesting and somewhat compelling.
Then part that I refer to is the underwater ruins found off the coast of India that may date back to a period far before recorded history. This is empirical and the age will eventually be determined. It doesn't sell ancient astronauts but the idea that there may have been a civilization over 10,000 years ago is fascinating.
To a lesser extent I have always been puzzled by the ground designs in the Andes that can only be interpreted from high in the air.
Ancient Astronauts