Well, I can't help but giggle a bit at the "For example many of the ancient cultures have structures that are man made, but we would have a hard time making them today even with our modern technology." You know, when I had classes about prehistory, I learnt one thing - just because we do not know how it's done, doesn't mean that the people who created it didn't know it as well, not to mention that people who do some craft can re-create it as well, given the right tools. The funniest example our teacher gave us was usage of stone axes. The archeologists were unable to cut a tree down, no matter how hard they tried - and when they finally managed to do it, it was so exhausting it wouldn't be really worth the energy, especially for prehistorian people. But then they gave the stone axes to professional woodcutters - and plop! Worked like charm.
I also think that one important thing in ancient times is that more than we now, they were "doomed" to use the trial and error method in probably everything they did. It probably wasn't written (or recorded in any other way) how many times some structure fell on its builders before they figured the perfect way how to build it. We just see the results, but now what actually led to them. And about the pyramids all around the world - well, it's quite proven fact that old civilizations had been able to travel quite far from the place they originated. Figures they would also carry their ideas about how things should be built with them, no?
Oh, and just to say it aloud - I used to be great fan of Däniken and his works. But after reading several of his books, some of his reasoning became so absurd to my eyes taht I'm unabe to take him at least a bit seriously anymore. Definitely says a lot about human race, if nearly everything about their civilizations had to be started by outside intervention, eh?