In the late Cretaceous era (70 million years ago) there was a species of therapod dinosaur resembling a large Tyrannosaurus, but with special amphibious adaptations which allowed it to swim between the islands upon which it hunted. This dinosaur species has been dubbed "Gojirasaurus".
A minimal breeding population of gojirasaurs somehow survived the great extinction event which killed off other dinosaurs and continued into the modern era. (Much the way many postulate that breeding populations of plesiosaurs survived to become modern "lake monsters".) Adaptations which aided the gojirasaurs in their survival probably included the ability to lay dormant for extended periods of time, and to endure high heat and radiation environments like active volcanic regions where man rarely encroaches. Living in the South Pacific, the gojirasaurs were observed only on rare occasions by the island people of simple fishing villages who incorporated the stories of these strange beasts into their folklore.
All but one (or perhaps two) of the gojirasaurs were apparently destroyed in World War II. One gojirasaurus was severely injured by Allied attack, but managed to cling to life on the remote island of Lagos (near the Marshal Islands in the South Pacific Ocean) until post-war nuclear tests caused him to be subjected to a huge dose of radiation. The radiation caused the gojirasaurus' already radiation-attuned physiology to rapidly mutate/adapt. The animal grew and changed until the gojirasaurus had developed into an entirely new form of creature... Godzilla!
After being seriously injured by American troops, Godzilla was left to die only to be mutated into a huge fire-breathing monster 10 years later by radiation from atomic tests in the Pacific. In the film, "Godzilla versus King Ghidorah," the details of the origin of Godzilla are visited. His origin was modified by time travelers from the future so that the wounded godzillasaurus was teleported to the Bering Sea, where an accident involving a nuclear submarine in the present day created a larger, more powerful Godzilla.
The Godzilla of 1998 in the Tri-Star movie Godzilla was also created by nuclear testing. Although in this film Godzilla was an enlarged iguana, not a sleeping dinosaur.
Some says Eiji inspired from the American film, "The Beast from 20,000 Fathoms." Other pretends that inspiration for Gojira came from the incident of Fukuryu Maru. In 1954, a fishing boat strayed in the waters around the Bikini Atoll atomic testing site. Returning to Japan, the crew was afflicted with a strange illness, one dying as a result. Panic swept through the country, and a recall of tuna was ordered because of the suspicion of contamination. The Japanese press labelled the incident 'the second atomic bombing of mankind.'" -- Markalite, Fall 1991
In some Godzilla films released in Germany before 1985, the storylines were altered so that many of Godzilla's opponents were either sent or created by Dr. Frankenstein.
Description of Godzilla