The Lost City of Pnakotus (also called the ''Library City'') is located in Australia's Great Sandy Desert. This primordial city is where the Great Race of Yith housed their enormous library. Today it is inhabited only by flying Polyps, or even stranger things.
The library of Pnakotus held The Pnakotic Manuscripts, a legendary tome containing a detailed chronicle of the Great Race's history, among other things. Copies of this manuscript would later be passed down through the ages, eventually falling into the hands of sinister cults which would guard them into modern times.
Pnakotus features in the Chaosium adventure City in the Sands, which appeared in Terror Australis and some versions of Masks of Nyarlathotep. There are various indications in DG fiction that Joseph Camp survived Masks and visited Pnakotus.
Pnakotus is the setting for the David Conyers' DG-like short story, The Impossible Object, available for free download from Chaosium as part of the promo for the Conyers/Sunseri book The Spiraling Worm (also very DG-like).
An interesting discussion of why outback Australia was a popular site for "lost civilisation romances" in Lovecraft's era, by Melissa Bellanta, is here. By kind permission, Melissa also appears as an NPC in The Fraternal Order of Librarians.