I read abook years ago called 'fingerprints of the gods' or something, it was about this kind of thing. The author was convinced all these ancient monuments such as the pyramids, machu piccu ? , etc, etc, were all either built by aliens or built for aliens.like i said, this was years ago, so my memory of it is quite hazy, it was a very convincing read though.
Have read that book as well and very plausible it is too, some very interesting theories
the 'alien' angle though seems very hard to believe, not saying thats not possible though
the scientific evidence though does suggest that that the pyramids are much older than conventional egyptologists originally thought, which is i believe about 3000 years old, more and more scientists are know saying the true age of the structures are nearer to 12000 years, which if true is pretty damn amazing
the one thing about that book that got me thinking, was the chapter about the so called 'Tomb' in the Kings chamber, a granite block that after careful examination had been drilled out with diamond tipped tubular drills! an engineering marvel even by todays technological standards!
so whether it was aliens or some forgotten civilisation, there is definately more to it than what we are led to believe!