There is considerable evidence showing that Columbus and even the Norsemen wern't the first explorers to set foot on the soil of the Americas. While this subject is still downplayed in the U.S. and certainly not included in school textbooks, there is colabraration on the subject from scholars and historians today, and even those writing back in ancient times. Going all the way back to Plutarch, who wrote that from the ruins of Carthage he discovered that, "those Greeks of North Africa had settled among the barbarbians of the western Epeiros". ( the continent that rims the western ocean?)
Of course, we know now that the ancient world knew of the earth was a globe, and even latitude and longitude were understood at least as far back as Eratosthenes. He (Eratosthenes) was able to calculate the circumference of the earth within 50 miles of what we know it to be today in circa 200 B.C.E.! I believe that this concept was known much eariler than that, but his appears to be the first written record. Not that the average farmer/heardsman understood this at the time, but it was well known to the educated of the Egyptians, Greeks, Lybians, Celts and others of the ancient world. Knowledge must have taken a back seat by the time of 1492, when Columbus sailed the ocean blue, and we were all taught in school the everyone,(back then) thought that the earth was flat!
Of course, knowing that the earth was round dosen't mean that anyone crossed the ocean in of itself. The evidence however isn't as obsure and untenable as it might seem at first glance, though the evidence has been thourghly dismissed by academia over the last hundred years. Those who have expertise in etymology and epigraphy have studied indian petroglyphs and asscoiate many of them with arabric, greek and celtic writing. Even a layman like myself can recognize certain astrological symbols common to the hellenistic world, not to mention a vast store of coinage that has been found from New England to South America that are Roman, Arabric,etc. Of course these items are usually said to be forgeries or just placed there to confuse the "truth", proclaimed by those who seem intent on the status quo of this subject.