In classical and medieval literature, ultima Thule (Latin "furthermost Thule" ) acquired a metaphorical meaning of any distant place located beyond the "borders of the known world".
By the late middle ages and early modern era, the Greco-Roman Thule was often identified with the real Iceland or Greenland. Sometimes Ultima Thule was a Latin name for Greenland, when Thule was used for Iceland.
The Greek explorer Pytheas of Massalia (now Marseille, France) is the first to have written of Thule, after his travels between 330–320 BCE. Pytheas mentioned doing Thule in his now lost work, Things about the Ocean ὰ περὶ τοῦ Ὠκεανοῦ (ta peri tou Okeanou). He supposedly was sent out by the Greek city of Massalia to see where their trade goods were coming from. Descriptions of some of his discoveries have survived in the works of later, often skeptical, authors. Polybius in his Histories (c. 140 BC), Book XXXIV, cites Pytheas as one "who has led many people into error by saying that he traversed the whole of Britain on foot, giving the island a circumference of forty thousand stadia, and telling us also about Thule, those regions in which there was no longer any proper land nor sea nor air, but a sort of mixture of all three of the consistency of a jellyfish in which one can neither walk nor sail, holding everything together, so to speak."
The 1st century BCE Greek astronomer Geminus of Rhodes claimed that the name Thule went back to an archaic word for the polar night phenomenon – "the place where the sun goes to rest". Dionysius Periegetes in his De situ habitabilis orbis also touched upon this subject as did Martianus Capella. Avienus in his Ora Maritima added that during the summer on Thule night lasted only two hours, a clear reference to the midnight sun.
Strabo in his Geographica (c. 30 CE), mentions Thule in describing Eratosthenes' calculation of "the breadth of the inhabited world" and notes that Pytheas says it "is a six days' sail north of Britain, and is near the frozen sea". But he then doubts this claim, writing that Pytheas has "been found, upon scrutiny, to be an arch falsifier, but the men who have seen Britain and Ireland do not mention Thule, though they speak of other islands, small ones, about Britain". Strabo adds the following in Book 5: "Now Pytheas of Massilia tells us that Thule, the most northerly of the Britannic Islands, is farthest north, and that there the circle of the summer tropic is the same as the Arctic Circle. But from the other writers I learn nothing on the subject – neither that there exists a certain island by the name of Thule, nor whether the northern regions are inhabitable up to the point where the summer tropic becomes the Arctic Circle." Strabo ultimately concludes, "Concerning Thule, our historical information is still more uncertain, on account of its outside position; for Thule, of all the countries that are named, is set farthest north." The inhabitants or people of Thule are described in most detail by Strabo (citing Pytheas): "the people (of Thule) live on millet and other herbs, and on fruits and roots; and where there are grain and honey, the people get their beverage, also, from them. As for the grain, he says, since they have no pure sunshine, they pound it out in large storehouses, after first gathering in the ears thither; for the threshing floors become useless because of this lack of sunshine and because of the rains".}}
In 77 CE, Pliny the Elder published his Natural History in which he also cites Pytheas' claim (in Book II, Chapter 75) that Thule is a six-day sail north of Britain. Then, when discussing the islands around Britain, he writes: "The farthest of all, which are known and spoke of, is Thule; in which there be no nights at all, as we have declared, about mid-summer, namely when the Sun passes through the sign Cancer; and contrariwise no days in mid-winter: and each of these times they suppose, do last six months, all day, or all night." Finally, in refining the island's location, he places it along the most northerly parallel of those he describes:
The Roman historian Tacitus, in his book chronicling the life of his father-in-law, Agricola, describes how the Romans knew that Britain (in which Agricola was Roman commander) was an island rather than a continent, by circumnavigating it. Tacitus writes of a Roman ship visiting the Orkneys and claims the ship's crew even sighted Thule. However their orders were not to explore there, as winter was at hand. Some scholars believe that Tacitus was referring to Shetland.
The 3rd Century, Latin grammarian Gaius Julius Solinus wrote in his Polyhistor that "Thyle, which was distant from Orkney by a voyage of five days and nights, was fruitful and abundant in the lasting yield of its crops". The 4th century Virgilian commentator Servius also believed that Thule sat close to Orkney: "Thule; an island in the Ocean between the northern and western zone, beyond Britain, near Orkney and Ireland; in this Thule, when the sun is in Cancer, it is said that there are perpetual days without nights..."
Other late classical writers and post-classical writers, such as Orosius (384–420) and the Irish monk Dicuil (late 8th and early 9th century), describe Thule as being north and west of both Ireland and Britain, strongly suggesting that it was Iceland. In particular, Dicuil described Thule as being beyond islands that seem to be the Faroe Islands (which are between Shetland and Iceland).
In the writings of the historian Procopius, from the first half of the 6th century, Thule is a large island in the north inhabited by 25 tribes. It is believed that Procopius is really talking about a part of Scandinavia, since several tribes are easily identified, including the Geats (Gautoi) in present-day Sweden and the Sami people (Scrithiphini). He also writes that when the Herules returned, they passed the Warini and the Danes and then crossed the sea to Thule, where they settled beside the Geats.
Another hypothesis, first proposed by Lennart Meri in 1976, holds that the island of Saaremaa (which is often known by the exonym Osel) in Estonia, could be Thule. That is, there is a phonological similarity between Thule and the root tule- "of fire" in Estonian (and other Finnic languages). A crater lake named Kaali on the island appears to be have been formed by a meteor strike in prehistory.  This meteor strike is often linked to Estonian folklore which has it that Saaremaa was a place where the sun at one point "went to rest".
In 2010, scientists from the Institute of Geodesy and Geoinformation Science at the Technical University of Berlin, claimed to have identified persistent errors in calculation that had occurred in attempts by modern geographers to superimpose geographic coordinate systems upon Ptolemaic maps. After correcting for these errors, the scientists claimed, Ptolemy's Thule could be mapped to the Norwegian island of Smøla.
In 1775, during his second voyage, Captain Cook named an island in the high southern latitudes of the South Atlantic Ocean, Southern Thule. The name is now used for a group of three southernmost islands in the South Sandwich Islands, one of which is called Thule Island. The island group became a British overseas territory of the United Kingdom , albeit also claimed by Argentina (in Spanish Islas Tule del Sur).
In 1910, the explorer Knud Rasmussen established a missionary and trading post, which he named Thule (Inuit: Avanaa) on Greenland. The Thule people, the predecessor of modern Inuit Greenlanders, were named after the Thule region. In 1953, Avanaa became Thule Air Base, operated by United States Air Force. The population was forced to resettle to New Thule (Qanaaq), 110 kilometres (67 mi) to the north ( only 840 NM from the North Pole).  The Scottish Gaelic for Iceland is Innis Tile, which literally means the "Isle of Thule".
Ultima Thule is the name of a location in the Mammoth Cave system in Kentucky, United States. It was formerly the terminus of the known-explorable southeastern (upstream) end of the passage called "Main Cave", before discoveries made in 1908 by Ed Bishop and Max Kaemper showed an area accessible beyond it, now the location of the Violet City Entrance. The Violet City Lantern tour offered at the cave passes through Ultima Thule near the conclusion of the route.
The Southern Thule islands were occupied by Argentina in 1976. The occupation was not militarily contested by the British until the 1982 Falklands War, during which time British sovereignty was restored by a contingent of Royal Marines. Currently, the three islands are uninhabited.
In March 2018, following a naming competition, the Kuiper Belt object 2014 MU69, a fly-by target of the NASA probe New Horizons was nicknamed "Ultima Thule". The pass is scheduled for 1 January 2019, and will be the most distant encounter between a spacecraft and a planetary body. An official name for the body will be assigned by the International Astronomical Union after the fly-by.
The Roman poet Silius Italicus (25 – 101 CE), who wrote that the people of Thule were painted blue: "the blue-painted native of Thule, when he fights, drives around the close-packed ranks in his scythe-bearing chariot". , implying a link to the Picts (whose exonym is derived from the Latin pictus "painted"). Martial (40 – 104 CE )talks about "blue" and "painted Britons", just like Julius Caesar. Claudian (370 – 404 CE) also believed that the inhabitants of Thule were Picts.
A work of prose fiction in Greek by Antonius Diogenes entitled The Wonders Beyond Thule appeared c. 150 CE or earlier. (Gerald N. Sandy, in the introduction to his translation of Photius' 9th century summary of the work, notes that this Thule most closely matches Iceland.)
Early in the 5th century AD Claudian, in his poem, On the Fourth Consulship of the Emperor Honorius, Book VIII, rhapsodizes on the conquests of the emperor Theodosius I, declaring that the Orcades "ran red with Saxon slaughter; Thule was warm with the blood of Picts; ice-bound Hibernia [Ireland] wept for the heaps of slain Scots". This implies that Thule was Scotland. But in Against Rufinias, the Second Poem, Claudian writes of "Thule lying icebound beneath the pole-star". Jordanes in his Getica also wrote that Thule sat under the pole-star.
The "known world' of the Europeans came to be viewed as bounded in the east by India and in the west by Thule, as expressed in the Consolation of Philosophy (III, 203 = metrus V, v. 7) by Boethius. "For though the earth, as far as India's shore, tremble before the laws you give, though Thule bow to your service on earth's farthest bounds, yet if thou canst not drive away black cares, if thou canst not put to flight complaints, then is no true power thine."
Ultima Thule (Thyle ultima) is an island of the Ocean in the northwestern region, beyond Britannia, taking its name from the sun, because there the sun makes its summer solstice, and there is no daylight beyond (ultra) this. Hence its sea is sluggish and frozen.
Isidore distinguished this from the islands of Britannia, Thanet (Tanatos), the Orkneys (Orcades), and Ireland (Scotia or Hibernia). Isidore was to have a large influence upon Bede, who was later to mention Thule.
By the late Middle Ages, scholars were linking Iceland and/or Greenland to the name Thule and/or places reported by the Irish mariner Saint Brendan (in the 76th century) and other distant or mythical locations, such as Hy Brasil and Cockaigne. These scholars included works by Dicuil (see above), the Anglo-Saxon monk Venerable Bede in De ratione temporum, the Landnámabók, by the anonymous Historia Norwegie, and by the German cleric Adam of Bremen in his Deeds of Bishops of the Hamburg Church, where they cite both ancient writers' use of Thule as well as new knowledge since the end of antiquity. All these authors also understood that other islands were situated to the north of Britain.
Eustathius of Thessalonica, in his 12th century commentary on the Iliad, wrote that the inhabitants of Thule were at war with a tribe whose members dwarf-like, only 20 fingers in height. The American classical scholar Charles Anthon believed this legend may have been rooted in history (although exaggerated), if the dwarf or pygmy tribe were interpreted as being a smaller aboriginal tribe of Britain the people on Thule had encountered.
A madrigal by Thomas Weelkes, entitled Thule (1600), describes it thus:
Hekla is an Icelandic volcano.
Thule is referred to in Goethe's poem "Der König in Thule" (1774), famously set to music by Franz Schubert (D 367, 1816) and Robert Schumann (Op.67, No.1), and in the collection Ultima Thule (1880) by Henry Wadsworth Longfellow.
By a route obscure and lonely,
Haunted by ill angels only,
Where an Eidolon, named Night,
On a black throne reigns upright.
I have reached these lands but newly
From an ultimate dim Thule –
From a wild weird clime, that lieth, sublime,
Out of Space – out of Time.
John Henry Wilbrandt Stuckenberg wrote on the subject in 1885:
What is the mind’s ultima Thule? What substance must be regarded as first, and therefore as the seed of the universe? What is the eternal Something, of which the temporal is but a manifestation? Matter? Spirit? Matter and Spirit? Something behind both and from which they have sprung, neither Matter nor Spirit, but their Creator? Or is there in reality neither Matter nor Spirit, but only an agnostic Cause of the phenomena erroneously assigned by us to body and mind?After spending many years in profoundly investigating this problem, I have at last struck bottom. Unhesitatingly and unconditionally I adopt materialism, and declare it to be the sole and all-sufficient explanation of the universe. This affords the only thoroughly scientific system; and nowhere but in its legitimate conclusions can thought find suitable resting-place, the heart complete satisfaction, and life a perfect basis. Unless it accepts this system, philosophy will be but drift-wood, instead of the stream of thought whose current bears all truth. Materialism, thorough, consistent, and fearless, not the timid, reserved, and half-hearted kind, is the hope of the world.— The Final Science: or Spiritual Materialism (1885) by John Henry Wilbrandt Stuckenberg (1835-1903), p. 6
"Ultima Thule" is a short story written by author Vladimir Nabokov and published in New Yorker magazine on April 7, 1973.
In Germany, extreme right occultists believed in a historical Thule, or Hyperborea, as the ancient origin of the "Aryan race" (a term which they believed had been used by the Proto-Indo-European people). The Thule Society, which close had close links to the Deutsche Arbeiter Partei (DAP), known later as the Nationalsozialistische Deutsche Arbeiterpartei (NSDAP or Nazi party) was, according to its own account, founded on August 18, 1918. In his biography of Lanz von Liebenfels (1874–1954), Der Mann, der Hitler die Ideen gab, Munich 1985 – The Man who Gave Hitler the Ideas, the Viennese psychologist and author Wilfried Daim claimed that the Thule Gesellschaft name originated from mythical Thule. In his history of the SA (Mit ruhig festem Schritt, 1998 – With Firm and Steady Step), Wilfred von Oven, Joseph Goebbels' press adjutant from 1943 to 1945, confirmed that Pytheas' Thule was the historical Thule for the Thule Gesellschaft.
Much of this fascination was due to rumours surrounding the Oera Linda Book, falsely claimed to have been found by Cornelis Over de Linden during the 19th century. The Oera Linda Book was translated into German in 1933 and was favoured by Heinrich Himmler. The book has since been thoroughly discredited. Professor of Frisian Language and Literature Goffe Jensma wrote that the three authors of the translation intended it "to be a temporary hoax to fool some nationalist Frisians and orthodox Christians and as an experiential exemplary exercise in reading the Holy Bible in a non-fundamentalist, symbolical way".