A legendary lost supercontinent in the Pacific, put forward by various writers including Colonel James Churchward and Erich Von Daeniken as an explanation for the seemingly abandoned megalithic architecture of places like Easter Island and Ponape, which were supposed to be the mountain-tops of the lost land.
In fact, most of these island based civilisations were feeble human attempts to recreate the lost glory of R'lyeh, inspired by the dream-messages of Cthulhu and contact with the Deep Ones. These dream-sendings drove Pacific Islanders to irrationally exploit their natural resources, and, having exhausted their food supplies and sanity attempting to erect non-Euclidian architecture, they turned on each other in orgies of cannibalism and bloodlust, supplying an ominous vision of the coming Endtimes to those with eyes to see.
Some legends recorded in the Ponape Scripture and the Zanthu Tablets refer to a specific sunken volcanic island continent, which may have been part of R'lyeh, inhabited by a Great Old One called Ghatanothoa. This being may in fact be Cthulhu or one of his Spawn. It was served and worshipped by humans, Lloigor, and possibly Mi-Go or some other race originating from Yuggoth. The land sunk when Ghatanothoa and another Great Old One called Ythogtha (Tsathoggua?) came into conflict. The legends suggest that Ghatanothoa was unable or unwilling to leave his tomb, possibly implying that R'lyeh had risen to the surface but the stars were not yet right for Cthulhu to awake.
This legend is in some way connected with the scandalous desecration of a mummy at the Cabot Museum of Archaeology in Boston, Massachusetts in 1932. Since P Division did not follow up the case, the truth will probably never be known.
It's also possible that some human legends referring to Mu may in fact refer to Sundaland, the sunken continental shelf of Southeast Asia. This case is argued in Stephen Oppenheimer's book, Eden in the East.