There's also the story of Prince Madoc of Wales, who sailed on the Gwennan Gorn in 1170. He supposedly discovered America, returned to Wales, gathered a fleet of ten small ships and some volunteers, and sailed to America again.
In the seventeenth or eighteenth century a tribe of Native Americans was discovered- The Mandans- who had white skins and spoke a language similar to Welsh. Some people believe that Madoc landed in Alabama and that the Mandans are the descendants of him and his volunteers.
In 1953 a tablet was erected on the shore of Mobile Bay, Alabama, which reads:
In memory of Prince Madoc, a Welsh explorer, who landed on the shores of Mobile Bay in 1170 and left behind, with the Indians, the Welsh language.
Near Penrhyn Bay, North Wales, there is a tablet inscribed:
Prince Madoc sailed from here
Aber-Kerrik-Gwynan, 1170 A.D.
And landed at Mobile Bay, Alabama
With his ships
Gorn Gwynant and Pedr Sant.
I have no idea whether or not any of this is true.