Hyperborea existed. That sentence is the whole problem with this file.

The continent is gone — buried under ice that has not retreated since the end of the Pleistocene — but the cities, the cults, the sorcerers, and the non-human worshipped things are documented, and the documentation is consistent across sources separated by millennia. The Pnakotic Manuscripts reference Hyperborean place-names that the Book of Eibon uses in identical context three thousand years later. Clark Ashton Smith, working in twentieth-century California from dreams he could not explain, produced a continental atlas whose bedrock features match modern subglacial radar surveys to a precision that no fabricator could have managed. The continent was real. The Bureau no longer argues this point internally.

What the Bureau argues is what to do with the file.

Geography

The Mu-Thulan peninsula was the primary settled region — a long, narrow landmass extending southward from the main continental body, temperate at its tip when most of Hyperborea was already cold. The capitals were built on Mu-Thulan because Mu-Thulan was the last region to lose its growing season. Commoriom came first: the jade-walled city, capital for an unknown number of centuries, abandoned after a prophecy delivered by a white-bearded oracle whose name has not survived. The official records describe the abandonment as orderly. Smith's notes describe it as a panic.

Uzuldaroum replaced it, built on a similar plan a few days' march to the south, with the same temples to the same gods and the same sorcerer-priests in the same offices. The continuity was maintained out of habit. The ice sheet to the north was already advancing during the period covered by the surviving Hyperborean records. Their civilization existed under an active deadline. They knew the ice was coming. It did not change their behavior in any documented way.

The Voormis

Pre-Hyperborean inhabitants. Hairy, primitive by the standards of their human successors, intelligent enough to maintain a coherent religion across geological time. They worshipped Tsathoggua long before the human settlers arrived from wherever the human settlers came from — and the Hyperborean records are vague on that question in a way that suggests the answer was uncomfortable.

When the settlers came, the Voormis retreated underground. Specifically, they retreated into the same caverns where Tsathoggua slept. The relationship became symbiotic in a sense the surface anthropologists never quite grasped. The Voormis maintained the rites in the deep places. Tsathoggua accepted the arrangement and did not object when the surface humans began offering supplementary worship of their own. The arrangement held for the entire Hyperborean era.

The Sorcerer-State

Hyperborea's government was not theocratic in the usual sense. It was a negotiated agreement between sorcerers — Eibon of Mhu Thulan the most notable, and the author of the field guide that bears his name — and the entities they had contacted on behalf of the state. The sorcerers provided prophecy, protection from rival cults, and the occasional documented miracle. The entities received worship, sacrifice, and uninterrupted access to the populations they fed on. The tax burden was distributed across the citizenry in a manner the surviving administrative tablets describe with bureaucratic precision.

Everyone understood the terms. There is no record of a Hyperborean reform movement. There is no record of a Hyperborean atheism. The entities were too obviously real for either response to make sense.

The End

Glaciation was the proximate cause. The deeper cause is debated within the Bureau and has been debated for as long as the file has existed. The Book of Eibon suggests — and Eibon was in a position to know — that the ice was a deliberate countermeasure. That Hyperborea had attracted attention from something older than Tsathoggua, something that came down out of the void in response to the sorcerer-state's escalating contact protocols, and that the only way to contain what the Hyperboreans had summoned was to bury the continent entirely. Eibon implies that the burial was negotiated with Ḋ̓͢R͕̮J̎̋͝A̶̧̧ꭥT̐͛I͆T̲̱̆R͡U͛T͓͛͢R͓̲P̍᳀ꫂ and paid for in a currency he does not name.

The evidence is circumstantial. The coincidence of timing is not. The glacial advance accelerated, geologically speaking, overnight.