10: The Seven Cities of Gold
Supposedly located somewhere in south western Continental North America, these legendary cities probably never existed outside of legends the early Spanish explorers were told by the natives. Essentially, the Seven Cities are the El Dorado of North America. There could be all kinds of things connected with them, especially why they fell into ruin, abandonment and eventually being forgotten.
Another thing is the connection of the Seven Cities to another, Portuguese legend of the legendary Island of Antilia, where supposedly a whole bunch of Visigothic Christian bishops fled to from the Muslim invasion of the Iberian peninsula, taking vast riches with them to eventually settle on an island somewhere in the Atlantic. In both cases there exist actual ancient documents and maps, but nothing concrete has ever been found, leaving the Seven Cities in the realm of legend.