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Old Earth is reference to the Earth which by the time in which Dune is set has long been forgotten. After the Butlerian Jihad, the Earth was devistated causing humans to leave Earth in order to settle on other planets.

There are several references to Old Earth woven throughout the Dune novels including Paul Atreides references to Khan and Hitler and LetoII made several references to memories he held of ancestors from various locations in Old Earth including a signer of the United States Constitution (Jacob Broom).