A popular theory that has been gaining moment in recent years revolves around the idea that long ago the Earth was visited by Aliens and that these advanced beings are responsible for some of the colossal building project usually attributed to ancient man. Proponents of this theory argue that ancient man simply did not have the technical know-how or the manpower to build such impressive structures as the Great Pyramids of Giza or the Nasca Lines in Peru. These proponents fail to understand the scientific principle of Extraordinary Claims require extraordinary evidence and Occam's Razor.
There is simply no evidence that aliens had anything to do with the building of the Great Pyramids of Egypt and loads of evidence that they were built by ancient man. Proponents of the theory think that just because the secrets of the engineering behind the pyramids was not expressively left behind by the Egyptians that this opens the door for alien construction.
Proponents also fail to realize that the simplest explanation is usually the correct one. What is more likely? Aliens came down from the sky and helped ancient man build stone monoliths or that ancient man build them as a crypt for their Pharaohs? What would have the aliens gained by building monument out of rock? Why not teach ancient man about something more interesting like steel construction or concrete? Proponents conveniently fail to answer these questions.
However, the theory does make for some interesting television. Just remember to apply scientific principles.