Thulium's name reaches back to classical geography and legend. Thule, first reported by the Greek explorer Pytheas around 300 BCE, was the farthest north any traveller claimed to have sailed, a half-mythical island of the frozen sea beyond which lay only the unknown; the phrase Ultima Thule came to mean the uttermost limit, the last place before the end of the world.
Cleve, a Swede naming an element of the far north, invoked this ancient boundary of the imagined earth. Thulium has no alchemical or religious tradition, being a nineteenth-century rare-earth discovery, but its name carries a genuine strand of ancient myth-geography, planting at the edge of the periodic table the island that once marked the edge of the world. Pytheas's own account of Thule does not survive directly; it is known only through later writers such as Strabo and Pliny who quoted and disputed him, some doubting he had sailed so far north at all, so that Thule entered the European imagination already wrapped in a layer of scholarly argument about whether it was Norway, Iceland, the Shetlands or pure invention. That inherited uncertainty suited Cleve's purpose well enough: he needed only a name evocative of remoteness, not a settled geography, when he reached for it to mark an element drawn from the far northern quarries of his own Scandinavian homeland.