Unexplained Structures

If ancient aliens visited Earth, can evidence of their existence be found in the mysterious structures that still stand throughout the world? Inexplicably, megalithic structures found on different continents are strikingly similar, and the cutting and moving of the massive stones used to build these magnificent feats would be a struggle for modern day machinery, let alone ancient man. Ancient astronaut theorists suggest that the standing stones in Carnac, France were used as an ancient GPS system for ancient flying machines. The recently discovered Gobekli Tepe in Turkey, which has been dated back 12,000 years, has finely chiseled pillars that experts describe as a Noah's Ark in stone. Is it possible that extraterrestrials assisted primitive man in constructing these unexplained structures? If so, what was the purpose of these grand projects?