A legendary Christian King who ruled the Orient between the 12th and 17th Centuries, Europeans believed in the existence of Prester John for more than 500 years... Letters were forged to prove the existence of this Oriental King who supposedly ruled over parts of India (or latterly Ethiopia, on an entirely different continent...) - and an area which contained the "Fountain of Youth", which if bathed in would restore a person to a youthful state.
Despite the complete lack of evidence to prove the existence of a Christian monarch named Prester John, Europeans completely bought the story of the mythical king - and Christopher Columbus cited this as one of the reasons he decided to pursue a life of exploration and discovery. Prester John was supposedly a descendant of the Three Magi - or three Biblical Kings - and his realm was said to contain the Gates of Alexander (a legendary barrier to keep the barbarians out to the north) and border the Earthly Paradise (the Garden of Eden).
Among the treasures he was also said to possess was a mirror through which every province in his kingdom could be seen. And people actually believed this for 500 years - just plain bizarre.