I believe rather strongly in the Ancient Astronaut theory. There's a great deal of very curious information and a lot of unusual archaeological data that tends to fall by the wayside when 'mainstream' archaeologists can't explain it. A lot of people roll out the explanations for the Nazca Lines and the Great Pyramids, which can be explained reasonably...although they ignore the fact that most experts in Egyptology quietly admit that they've never found an actual mummy dating back to Egypt in the Great pyramid (the mummies found date to Greek and Roman times). They also gloss over the fact that the only record linking the existence of the Great Pyramid to an Egyptian Pharaoh is linked back to a con undertaken by a disreputable British army officer.
Other than that we can say that the Egyptians built many pyramids, but there is nothing that explains the construction of the pyramids of Giza, the largest and the smallest of the Great pyramids aren't dedicated to any particular pharaoh.
There are myriad other unusual events and aspects in history that are glossed over, evidence in ancient Indian cities in the Indus valley showing they were destroyed by Atomic weapons (central glassed/fused land in the geographical center of the cities, bodies lying dead and showing signs of death from radiation poisoning), texts of Babylonian history that give explanations of 'weapons of the gods' that resemble the affects of nuclear weapons (massive explosions, slow painful death for those close by, warnings to not venture close to where the weapons detonated), Mesoamerican cultures believed to hold a common ancestor in their language that is completely without historical evidence.
Developmental cycles in the Middle east that showed rapid development followed by slow regression for three thousand years before suddenly making massive leaps once more, ancient civilizations showing knowledge of our solar system which we only recently developed (The ancient Babylonians knew about every major planet after Mars, we only know about them because of mathematical extrapolation of their existence.), development of cereal crops and agricultural processes in the Middle east in an eye blink (We take corn, grain, grapes, apples, and olives for granted, they were selectively bred over thousands of years to look that way, while other food crops seemed to just appear without precursor).
I could go on about castles in Northern England that demonstraight evidence of being struck by some kind of heat ray (solid stone being melted and fused in place), I could talk about identical existence of the creation mythology in every ancient religion, I could bring up the Babylonian Creation epic which describes humans being created by the gods using phials, beakers and jugs (the -only- detailed example of such a creation of mankind.) I could bring up the fact that every major religion has a polytheistic origin (even Christianity), I could speak of the cave paintings that describe their gods in ways that we consider the most common appearance of aliens. There are myriad examples of things that don't fit, things that are strange and things which are completely ignored.
There is a lot of information out there, and only a miniscule portion of it actually has anything to do with gigantic structures. A lot of it has to do with the sophistication of the civilizations and the archaeological evidence that's anomalous. I can say that sure, the ancient egyptians might have been able to build the pyramids (unlikely), or that the Incas built Macchu Picchu (nearly impossible), but saying that the ancient residents of the Indus Valley had nuclear weapons is obscene, just like saying that the ancient Picts had heat rays, or that the Babylonians knew how to genetically engineer cereal crops.
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