The Serpent Mound is located in Adams County, Ohio, on the outside edge of a magnetic meteor crater. One of the rarest elements on earth, iridium, can strangely be found in abundance in the grounds of The Serpent Mound. Iridium’s radioactive properties are highly valuable in outer space, leading ancient astronaut theorists to believe The Serpent Mound was actually built as a mining site for aliens. What do you think of this explanation? Tell us below. — at Serpent Mound.