"Geological and biogeographical studies of R'lyeh suggest that the landmass originally split off from the ancient supercontinent of Pannotia near the end of the Cryogenian Period approximately 650 million years ago, and has remained isolated from the continents since Precambrian times. As a result, much of R'lyeh's endemic life-forms derive from lineages otherwise known only from the early fossil record. While at first, the geographical remoteness of the landmass was what allowed its native organisms to flourish, after a time it was likely the alien nature of its life that enabled them to resist colonization from the mainland. [...]
R'lyeh has no native human population and for good reason: the island is located some twenty-five hundred kilometres from the nearest landmasses and lies within a vast oceanic desert at an extreme southern latitude. While it has been historically argued that some European and Polynesian voyagers may have encountered the lost corpse-city before its formal discovery in 1926, as far as it is known, no humans have ever lived on the island. It may be just as well, because R'lyeh's environment is often considered hostile to most non-endemic life. Competition between indigenous life concentrated on the relatively small, isolated landmass for aeons has accumulated into a fierce biological arms race. On the first documented voyage to the island in February of 1926, ten men were killed within the span of three days, arriving unprepared for what lay within.
The discovery and knowledge of R'lyeh came at a horrific human cost, as dozens of explorers have been sacrificed in the study and documentation of the island since then. [...]"
--- Passage from Field Guide to R'lyehian Flora and Fauna (Page 8, "Introduction")