Pytheas was a Greek geographer from the fourth century BC. He lived in Massilia (present-day Marseilles), and is famous for a voyage he made to the north.
On this voyage, he traveled to Great Britain, and possibly circumnavigated it. He also mentions a land called Thule which was even further north. Whether Thule was Scandinavia, the [[Shetland Islands]] or Iceland is not known, it is also not sure whether Pytheas actually visited Thule or only heard about it. He may have visited the North Sea coasts, including the amber island of Helgoland, and possibly even the Baltic.
Unfortunately, the book that Pytheas wrote about his travels has been lost, and all we know about him comes from later Greek and Roman commentators, who were very critical of him and did not believe he actually visited the lands he claimed to have visited. Nevertheless, the details that these commentaries mention were enough to show later historians that he actually did.