There are a few descriptions ascribed to Pytheas that deal with “Thule”:

In his travel descriptions Pytheas claims that he reached Thule six days after surrounding Albion/ Great Britain (Strabo 1.4.2. and Pliny nat.hist. 2.186). Thule is described as “near of the frozen sea”. In Pliny (4.104, 4.30) you can find additional references to Pytheas and Thule, that record that there was frequent travel between the island of Berrice and Thule. According to Strabo the “frozen sea” is in a distance of one day of travel to Thule, the same distance is recorded at Isidor of Charax. The frozen sea is called “mare concretum” or “Cronium” in Latin and Πεπηγυῖα θάλασσα in Greek.

Furthermore – according to Strabo (2.4.1.) air and water are not separated but mixed near Thule, like the “lungs of the sea”, without a possibility to go through. The meaning of this expression is still unclear. It could connect to an animal called πλεύμονες (as recorded at Plato and Aristotle) or refer on a phenomenon that occurs when the ice is melting and a layer of freshwater is covering saltwater and ice.

Finally, there is the account of the absence of night around the summer solstice (Pliny 4.104) and that this day is lasting for 6 months (Pliny 2.186).

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Thule became the symbol for the northernmost point of the world and there are a lot of different localisations: Iceland, Greenland, Faeroe Islands, Shetland Islands, somewhere in Scandinavia like Finland, Norway, …Eratosthenes (and based on him Strabo) gives a location for Thule that could mean Iceland, Greenland or the coast of Norway.

In 825 an Irish monk – Dicuil – is the first to locate Thule at Iceland but there are two arguments against it: Thule is described as being populated by people and bees and Iceland seems to have been inhabited by both much later.

Another problem is the possibility that Thule for the Romans is not the Thule that Pytheas meant and the same applies for the island of Berrice, the starting point for the trip to Thule by Pytheas.

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English translation and compilation of the preserved references to Pytheas:

Pytheas of Massalia: On the Ocean, transl. and commentary by Christina Horst Roseman, Chicago: Ares Publishers 1994

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Literature for the different theories of localisation (also in English):

Barry Cunliffe, The Extraordinary Voyage of Pytheas the Greek. New York: Penguin Books 2003

Duane W. Roller: Through the Pillars of Herakles. Greco-Roman exploration of the Atlantic. New York: Routledge 2006

William Stevenson: Robert Kerr's General History and Collection of Voyages and Travels, Volume 18. Historical Sketch of the Progress of Discovery, Navigation, and Commerce, from the Earliest Records to the Beginning of the Nineteenth Century, EBook-Release 2004