It's an image famous -- and controversial -- all the world round: in the Black Hills of South Dakota, the US government wrought four gigantic sculptures of past US Presidents on the face of the Six Grandfathers. In tonight's episode, Ben, Matt and Dylan explore the truth behind one of America's weirdest conspiracies: Is Mount Rushmore just a big carving? Or is there a secret, hidden for centuries, there within the stone?

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