To the ancient Greeks, Thule was the northernmost habitable region of the world. One of the first recorded sea journeys in the Atlantic was by Pytheas of Massalia, who sailed to England in about 330 B.C. from his Greek colony in what is now Marseilles, France. He wrote about his voyage in a book called About the Ocean of which no copies exist, but he is quoted in other works. Pytheas visited Britain, where tin was traded, and possibly Ireland, the Hebrides, and the Orkneys. Then he went to "Thule" which was described as the outermost of all countries, where the night was very short- 2 or 3 hours long. It could be reached by sailing north from Britain for six days. Pytheas was told that north of Thule the sea was solid and there was no night in the summer; and in this area, the ocean is a kind of substance resembling "sea-lungs" (jellyfish) where the earth, sea and air all blend together (fog). Pliny adds that one day's sail from Thule is the Cronian Sea (the frozen ocean.)
On maps, Thule (or sometimes Tile or Ultima Thule) usually appears north or northwest of England and Ireland or in the northernmost parts of Asia. Later explorations revealed that it didn't actually exist, but It has been associated with early reports of Iceland, or the Shetland Islands, but could have been Trondheim in Norway as well.