The Earth's hollow and Hitler found an entrance to it in the south pole and escaped there at the end of world war 2. Ancient aliens also landed and posed as gods early in our history too which explains why most religions have the same general facts and most gods are depicted/described in basic terms that could be used to explain space ships and the like.
There's some real interesting stuff out there. I don't believe any of them but I don't rule them out as crackpot either.
I do believe we were more technological in the past than what is recorded these days. There's the common belief that because we're more modern we must be so much smarter and our ancestors were dumb but that's not really true. There's alot of historical artifacts that don't fit with the time period that their dated from.
I did a history minor, and one of my history teachers once talked about this theory briefly.
He explained that he didn't think that this theory was plausible, but he also explained that the period between the Fall of Rome (approx 350 A.D.) and the start of the Christian crusades (approx 1000 A.D.) is a bit of a mystery in terms of what was happening in Europe. The amount of sources of information available from this period is ridiculously low compared to the ages before and after it. This of course doesn't mean that this period didn't exist, but he understood why this gap of information could lead to conspiracy theories. He probably exaggerated a bit, but he said we know more about what was happening in Europe during some single decades of the Roman era than we do about what was happening in Europe during some complete centuries of the period +/- 350-1000 A.D. In terms of information, the period between the Roman era and the start of the crusades is a bit of a blur.
Hope I got the point across a little, English is not my native language.