Nazca Lines
In the 1985 "Challenge of the GoBots" episode "Search for the Ancient Gobonauts", "the Evil One", is retroactively a version of Megatronus, according to "Ask Vector Prime". He nearly destroyed Gobotron with a device called the Dark Heart, only to wind up under the Nazca Lines on Earth. Centuries later, Cy-Kill goes on a quest to look for the ancient villain.