There is a book that was published not too long ago called "1421: The Year China Discovered the World" (however, the US edition was titled "1421: The Year China Discovered America"). It was written by Gavin Menzies, a former submariner.
He argues that during the Ming dynasty, there were these massive treasure fleets that sailed around the world. These ships were much larger than the ones Columbus sailed to America with. This fleet was commanded by a man name Zheng He, he was a Eunich serving as a chinese Admiral. He is also a muslim and was originally from Arabia.
Anyways.....In the book, there are maps drawn by the Chinese cartographers that traveled on this fleet of treasure ships. The goal of this fleet was to show the entire world the might of the chinese navy. This would legitimize chinese authority over other nations, which can lead to tributary rewards. These maps were drawn in 1421 and it is quite possible that some of these maps have been shared to other peoples of the world via trade. China at the time was the most developed nation and had the most vast trading networks, so it is likely. However, at around the time of the fleets return, the dynasty had changed it stance on its navy. The emperor had died, and was replaced by his son, who do not share his vision of world domination. He was more concerned with domestic affairs. This led to a book burning, which burned much of the discoveries of Zheng He's fleet.
But it is still quite possible that some of these maps have been distributed before its destruction.
However, I must admit, Gavin Menzies does stretch his hypothesis a little too much at times. And at best is just a conspiracy theory. It still warrants a good reading because it's quite a different perspective to take.
