In 1959, by his own account, Menzies was an officer on HMS Newfoundland on a voyage from Singapore to Africa, around the Cape of Good Hope, and on to Cape Verde and back to England.

He often refers back to his sea-faring days to support claims made in 1421. Menzies claims he sailed the routes sailed by Ferdinand Magellan and James Cook while he was commanding officer of the diesel submarine HMS Rorqual between 19, a contention questioned by some of his critics. From 1959 to 1970, Menzies served on British submarines. He never attended university and had no formal training in historical studies. Menzies dropped out of school when he was fifteen years old and joined the Royal Navy in 1953. He was educated at Orwell Park Preparatory School in Suffolk, and Charterhouse. Menzies was born in London, England, and his family moved to China when he was three weeks old. Photograph of HMS Newfoundland, on which Gavin Menzies claims to have been stationed as an officer in 1959. In his third book, The Lost Empire of Atlantis, Menzies claims that Atlantis did exist, in the form of the Minoan civilization, and that it maintained a global seaborne empire extending to the shores of America and India, millennia before actual contact in the Age of Discovery.

Menzies' second book, 1434: The Year a Magnificent Chinese Fleet Sailed to Italy and Ignited the Renaissance, extended his discovery hypothesis to the European continent. He was best known for his controversial book 1421: The Year China Discovered the World, in which he asserts that the fleets of Chinese Admiral Zheng He visited the Americas prior to European explorer Christopher Columbus in 1492, and that the same fleet circumnavigated the globe a century before the expedition of Ferdinand Magellan. Historians have rejected Menzies' theories and assertions and have categorised his work as pseudohistory. Rowan Gavin Paton Menzies (14 August 1937 – 12 April 2020) was a British submarine lieutenant-commander who authored books claiming that the Chinese sailed to America before Columbus. Who Discovered America?: The Untold Story of the Peopling of the Americas (2013).The Lost Empire of Atlantis: History's Greatest Mystery Revealed (2011).1434: The Year a Magnificent Chinese Fleet Sailed to Italy and Ignited the Renaissance (2008).1421: The Year China Discovered the World (2002).

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