I think Romans knew of America through the Carthaginians, who had colonies there. So did the Phoenicians, which is why Vikings also went there.
America was never really a new continent, it was a settled trading post since the times of Atlantis, countless ciivlizations came and went there. Copper mined in Michigan has been found in many places on Earth in great quantities dating back millennia, for just one example. Sepehr made a video about it.
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