And oral history I'm sure. We will probably never know how advanced and widespread some of these ancient technologies are but the existence of artifacts that are far, far ahead of anything we would expect historically (e.g. Antikythera Mechanism, Gobekli Tepe) suggests there are some big, big holes in our records of the past.
I don't think anybody really thought much about it before the discovery of the Antikythera Mechanism, but at least in retrospect there seem to be clear reference to such devices in ancient contemporary literature. Particularly, Cicero described such devices and attributed them to Archimedes.
I find the ancient stone works the best evidence for some kind of advanced civilization in the far past. (No not aliens). Some date back 10.000 years ago. The precision and similarities between different sites all over the world is incredible. We are just in the beginning of this research field.
