Presuming that such a civilization did exist, another important point as to why there are no artifacts found could be that they aren't on the surface anymore.

Presuming that such a civilization did exist, another important point as to why there are no artifacts found could be that they aren't on the surface anymore. Plates shift and subduct. Huge sections of land suddenly (geologically speaking) find themselves under the ocean. Just read about "Doggerland", or "Beringia", and you realize that the history we have is only the history of things that remain on the surface of the earth.