A lovely example of the Mercator-Hondius map focused on East Africa and the mythical Kingdom of Prester John, and including a large inset of the Congo with a detailed depiction of the known west-central African river system.
Prester John was a legendary Christian patriarch, presbyter (elder) and king who was popular in European chronicles and tradition from the 12th through the 17th centuries. He was said to rule over a Nestorian (Church of the East) Christian nation lost amid the Islamic caliphates and pagan lands of the Orient, in which the Patriarch of the Saint Thomas Christians resided. The accounts are varied collections of medieval popular fantasy, depicting Prester John as a descendant of the Three Magi, ruling a kingdom full of riches, marvels, and strange creatures.
Two lakes, Zaire and Zarian, are shown as the source of the Nile.