Just got done reading Gavin Menzies book 1421 The Year China Discovered America. Gavin was in the Royal Navy and sailed the world from 1959 to 1970 as part of the Royal Navy. The exhaustively researched conclusions in the book have the potential to re-write the history of who discovered America, Australia, New Zealand and Central America!
The contention is that between 1421 and 1423 great Chinese treasure fleets sailed the worlds oceans. When the fleets returned home, they returned to find China changed forever. On may 9th 1421 lightning struck the Forbidden City and the resulting fire convinced the emperor that the gods demanded change. The resulting turmoil set China on an inward course. Too ensure that no more epic voyages took place, not only were the shipyards put out of commission but the plans and the accounts of the completed voyages were deliberately destroyed.
What did survive of Chinese cartography from the voyages effectively lead the Portuguese to the east. The descriptions of maps, charts and currents really bring this story to life like only a sailor could tell it. It also explains why other historians would have had a hard time following the chain of evidence about the 1421 voyages that Gavin did. I highly recommend this book. You will definitely question the history of Dias, Columbus, Magellan and Cook learned in school.