Yuggoth is the outermost world of the solar system in Lovecraft's fiction, first described at length in "The Whisperer in Darkness." It is depicted as a place of black rivers and vast fungoid cities, serving as the base from which the Mi-go conduct expeditions to Earth.
Shortly after Pluto's discovery in 1930, Lovecraft explicitly identified Yuggoth with the new planet. The name also titles his sonnet-cycle "Fungi from Yuggoth."