The 1421 hypothesis suggests that during the Ming Dynasty, from 1421 to 1423, ships commanded by the Chinese captains Zhou Wen (周聞), Zhou Man (周滿), Yang Qing (楊慶) and Hong Bao (洪保), in the fleet of Emperor Zhu Di's (朱棣) Admiral Zheng He (鄭和), travelled to many parts of the world unknown to contemporary Europe. The suggestion was put forward by former British Royal Navy submarine commander Gavin Menzies in his book, 1421: The Year China Discovered the World, first published in 2002.