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Sacsayhuamán
Only a few miles east of the Inca capital, Cusco lies a stone fortress that had perplexed archeologists for years. Amateur scientists theorized that these several thousand-pound stones were placed there by alien life that was visiting earth thousands of years ago. Oddly, this became an alarmingly common rumor and went unaddressed for years until archeologists dug up the remains of a complex rope and lever system that was used to transport the stones up the hill. Give humans a little credit eh?
Nasca Lines
In the Peruvian desert, there are these large lines, carved into the white limestone that is under the sand layer of the desert. These lines intersect in incredibly precise ways to draw these enormous figures into the landscape. Conspiracy theorists believe that these lines were drawn by aliens because humans did not have the ability to fly until later on in history. This was later proved unnecessary, as the ancient Peruvian people, who were adept at mathematics and theory, created these ritualistic sand drawings using an algorithm that required great mathematical skill to complete.
Egyptian Pyramids
This one is probably the most ridiculous. There is a grand hypothesis between conspiracy theorists that suggests that the pyramids were built by aliens, thousands of years ago, to send microwaves into alien space ships to power them. So much is wrong and weird about this theory, but there has recently been a breakthrough in archeological exploration of the site of the pyramids. The Egyptian pyramids have perplexed historians since they were discovered, nobody knew how they were built until recently. Turns out, there were canal systems built into the area surrounding the pyramids, which people carried the stone blocks through on boats.