I bet aliens built those things.........they say that they have found skeletons of giants........the type you see in Avatar..........

Were Giant Skeletons Discovered at Serpent Mound? - Facebook

https://www.facebook.com/notes/hugh-newman/...at...mound/10154136787440948/

Ross has written several books about Serpent Mound including Star Mounds: ... The proposed Ohio Valley mound and star correlations, by Ross Hamilton.