There are no records of Hellenes going beyond Sahara. Even the Ancient Egyptians did not do that in their couple of thousand years of existence. The Phoenician admiral Hanno most likely reached what is now Gulf of Guinea and Mt. Cameroon, right by Nigeria (actually even farther than Nigeria, Mt Cameroon is in Cameroon). That is the farthest the Phoenicians probably ever sailed.
The farthest and the most unbelievable voyage of a Hellene, in my opinion was of the Massalian Greek named Pytheas. According to his account he rounded Spain, proving it was a peninsula, reached Brittany (modern-day France) and eventually the famed Tin Isles themselves, where he was greeted by Celts who showed him the tin mines. Pytheas then proceeded to reach the tip of moder-day Scotland and then sailing North until reaching Thule, which was most likely to be Iceland, according to Pytheas' description ("the ever-shining fire" of the "immense summit" - most likely the volcanoes of Iceland, since there are no active volcanoes on either the Shetland or Faroe Islands). Pytheas described the "midnight sun" as well as the peculiar sea sludge "neither jelly, nor water, nor earth" on which "one could neither walk nor sail" which is a type of ice sludge that forms only near the Poles. After reaching Iceland he sailed by the Scandinavia, near modern-day Norway and landed in Denmark, "the Amber Isle" too. From this point on he took pretty much the same way home. All of this time in 75-100 ton cargo trading ship. He left the "memoirs" of his voyage, titled "The Ocean" but unfortunately it did not survive to our times. His work was however widely quoted, so we are not entirely clueless. Pytheas' accounts were long dismissed as fanciful and false tall-tales until recently.
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