Mu is an ancient underwater kingdom which lies triangulated midway between Hawai'i, Japan, and the Philippines. It is inhabited by a race of blue-skinned, water-breathing merfolk.

The last known King of Mu was Te Ari'ikinui Ra'a, also known as the superhero Seamaster. Seamaster and the elite Royal Guard of Mermen were all killed in 1946 by the atomic bomb blast over Nagasaki, Japan.

There has been no official contact with Mu since 1945. Some people in the public now claim it was a hoax or myth created by the United States government during wartime.