Outwardly identical to humans, the Seatopians built their civilisation both beneath the waves and under the sea long before the coming of Man. They live in vast subterranean metropolises, are highly technologically advanced, and favour a Greco-Roman style in both their architecture and the way that they dress. The kaiju Megalon is worshiped as a god by the Seatopians.
For many long years the Seatopians watched as Humanity slowly destroyed the planet, waiting in the hope that things would get better. The final straw came however, when the same nuclear test that destroyed Monster Island simultaneously laid waste to their capital, the city of Seatopia.
Realising that Mankind would never change its ways, they called upon their god Megalon to exact revenge. Also taking control of the humanoid robot Jet Jaguar, and enlisting the aid of the space monster Gigan, the Seatopians invaded the surface world, but were ultimately defeated by the combined efforts of Humanity and Godzilla.
Ever since then, the Seatopians have kept themselves hidden away, although they continue to scheme beneath the waves.