R'lyeh is a fictional lost city that first appeared in the H. P. Lovecraft short story "The Call of Cthulhu", first published in Weird Tales in June 1928. In the story, R'lyeh is a sunken city in the South Pacific and the prison of the entity called Cthulhu.
The nightmare corpse-city of R'lyeh…was built in measureless eons behind history by the vast, loathsome shapes that seeped down from the dark stars. There lay great Cthulhu and his hordes, hidden in green slimy vaults.
— H. P. Lovecraft, "The Call of Cthulhu" (1928)