A civilisation which had existed for thousands of years, Seatopia and its people lived a life of peace in underground cities akin to ancient Roman and Greek civilisations. However, from the late 1960's onwards, Seatopia was repeatedly damaged by nuclear tests carried out by the nations above. The final straw came when a nuclear test wiped out Monster Island. Calling upon their god, Megalon, the Seatopians prepared an attack on those who were unknowingly destroying them. They also sought assistance from the M Space Hunter Nebula Aliens, and acquired Gigan. The two monsters were set loose upon the Japanese countryside. After the monsters fled from the might of Godzilla and Jet Jaguar, the Seatopians chose to seal Seatopia underground once more, but vowed to return if their cities came under threat again.