Mu is a supposed lost continent that was said to have sunk beneath the waves of the Pacific Ocean. This pseudo-scientific geologic history has been incorporated into the Cthulhu Mythos.
In the real world[]
Mu is the name of a suggested lost continent whose concept and name were proposed by 19th-century traveler and writer Augustus Le Plongeon, who claimed that several ancient civilizations, such as those of Egypt and Mesoamerica, were created by refugees from Mu—which he located in the Atlantic Ocean, and identified with Atlantis.
This concept was popularized and expanded by James Churchward, who asserted that Mu was a different lost continent than Atlantis; he located Mu in the Pacific Ocean.
H. P. Lovecraft (1890–1937) featured the lost continent in his revision of Hazel Heald's short story "Out of the Aeons" (1935). Mu appears in numerous Cthulhu Mythos stories, including many written by Lin Carter.
In the Mythos[]
It's been claimed that the first human beings inhabited Mu (CIRCLE: "The Invaders"), although other sources claim that they inhabited Lemuria (HPL: "The Haunter of the Dark"), perhaps hinting at some connection between the two continents.
The Muvians mainly worship the ghastly Ghatanothoa (trapped under mount Yaddith-Gho), though there are some who oppose this prevailing religion. Among them are T'yog the high priest of Shub-Niggurath (HPL: "Out of the Aeons") and Zanthu the hierophant. (EXP: Xothic legend cycle, Lin Carter) In trying to free his god Ythogtha and restore his cult, Zanthu accidentally causes the destruction of Mu. (EXP: "The Thing in the Pit" [Lin Carter])
It's also believed that between 18,000 and 10,000 BC, the Muvians were enslaved by extraterrestrial creatures called the Lloigor, led by Ghatanothoa. (EXP: "The Return of the Lloigor")
At some point in history, the people of Mu waged wars against the Atlanteans. (HPL: "Bothon")