There is a persistent legend that a Welsh prince named Madoc arrived in North America in 1170. This story was retold in 1580 to persuade Queen Elizabeth I of England that much of North America belonged to the British.
Explorers who visited or lived with the Mandan people of North Dakota, including the Lewis and Clark expedition in 1804-05 and nineteenth-century painter George Catlin, reported that their language was so similar to Welsh that the Mandan people understood it. However, a Welsh traveller named John Evans spent the winter of 1796-97 with the Mandans while searching for the “Welsh Indians”. His conclusion? There weren’t any. If Lewis and Clark found Mandans who understood Welsh, the Mandans had probably learned it from Evans eight years earlier.
It is harder to explain why, prior to known European contact, so many Mandans were light skinned and had brown, blonde or red hair and blue or hazel eyes, instead of having black hair, brown eyes and darker skin like their neighbours. Some people suggest a Norse influence, but the land of the Mandans was a long way from the coastal waters of the Atlantic.
An analysis of Mandan DNA might solve this mystery, but the Mandans were nearly wiped out by smallpox in the 1800s. The few survivors were adopted into other tribes, and intermarriage with those tribes and with people of other nationalities makes it impossible to arrive at an answer using DNA from living Mandan descendants, if any could be found.