This old guy says he has evidence that there were people here before Columbus, a Prince Madoc, and King Arthur (I think he says King Arthur was here...).
This link, http://www.bbc.co.uk/blogs/waleshistory/2010/... , explains it in more detail.
"The man in question was Prince Madoc, the son of Owain Gwynedd, one of the greatest and most important rulers in the country, and while the legend cannot be corroborated there are many who believe it implicitly. Owain Gwynedd certainly existed, his reign being marred by long and hard-fought disputes with Henry II, king of England.
The story goes that in 1170 Owain died and, almost immediately, a violent and very bloody dispute arose between his 13 children regarding the succession. Madoc and his brother Rhirid were so upset and angered by events that they decided they wanted no further part in what was happening. Indeed, they wanted nothing more to do with their family or their homeland. They duly took ship from Rhos on Sea (Llandrillo) and sailed westwards to see what they could find.
What Prince Madoc found, so the legend runs, was America. He and his brother managed to cross the Atlantic and land on the shores of the New World. Madoc returned to Gwynedd for more men, then sailed off again, this time never to return. His sailors inter-married with a local Native American tribe and for years the rumour of Welsh speaking Native American tribes was widely believed."
Wikipedia says of Madoc:
There have been suggestions that the wall of Fort Mountain in Georgia owes its construction to a race of what the Cherokee termed "moon-eyed people" because they could see better at night than by day.(A competing tradition claims that the wall was built by Hernando de Soto to defend against the Creek Indians around 1540.) Archaeologists believe the stones were placed there by Native Americans. These "moon-eyed people," who were said to have fair skin, blonde hair and opalescent eyes, have often been associated with Prince Madoc and his Welsh band. Benjamin Smith Barton proposed that these "moon-eyed people" who "could not see in the day-time" may have been an albino race.