The craziest thing is there is some evidence that ancient Celts, and perhaps Carthage had visited North America. There are a few tribes with languages that are similar, some traces of an old Celtic writing system, and other links.

For decades academia wrote it off as quackery of one guy, but as time has gone on more people have accepted it might have happened.

If true, that makes the stone age stuff even more sad, because it would mean at some point they had access to greater technology, but somehow lost it.