Greek knowledge of the outside world was heavily based on hearsay; Greeks loved fantastical stories about lands beyond their immediate knowledge, and so even great historians like Herodotus reported that India was populated by magical beasts and by men with multiple heads. In turn, the immediate knowledge Greeks actually had of the world extended to the coasts of the Mediterranean, the Black Sea, Egypt, and Persia, since those were the areas they had colonized or were in contact with through trade. Through the Classical Era, a strong naval garrison was maintained by the Carthaginians, Phoenician naval rivals of the Greeks, at the straits of Gibraltar (the narrow gap between North Africa and southern Spain between the Mediterranean and the Atlantic Ocean), which prevented Greek sailors from reaching the Atlantic and thereby limiting their direct knowledge of the world beyond.
One exception to these limited horizons was a Greek explorer named Pytheas. A sailor from the small Greek polis Massalia that was well-known for producing ship captains and navigators, Pytheas undertook one of the most improbable voyages in ancient history, alongside the famous (albeit anonymous) Phoenician voyage around Africa earlier. Greek sailors already knew the world was round and had devised a system for determining latitude that was surprisingly accurate; Pytheas’ own calculation of the latitude of Massalia was only off by eight miles. Driven by a sense of how large the world must be, he set off to sail past the Carthaginian sentries and reach the ocean beyond.