R'lyeh
From Arkham Horror Wiki
R'lyeh is an Other World from the Arkham Horror base set. It is a sunken city theoretically located deep under the Pacific Ocean, and is ruled by the Ancient One Cthulhu.
List of other world encounters
Mythos Source
R'lyeh first appears in H.P. Lovecraft's short story, The Call of Cthulhu (1928).