The Dark Island is a lush tropical island off the coast of Chile. The island is approximately 15 miles across, and in 1965 was the site of a nuclear experiment by the U.S. Military. The military scientists were experimenting with nuclear fusion as a power source, since it would be much more efficient than fission. However, there was a failure that caused the necessity to evacuate the island and all its facilities. Radiation escaped, so the whole complex was covered in concrete.
However, minute amounts were able to escape and affect the life surrounding it. Worse still, the island had at the same time been the site of a biological experiment involving growth hormones. Many exotic reptiles, birds, plants, insects and mammals had been brought there to have the hormone tested on them. However, the radiation caused the hormone to go out of control, and the plants and beasts grew to megaproportions, and some even went berserk, destroying the research facility and killing the scientists.
Later, in 2003, several "Martians" in the guise of explorers came to the island and saw the creatures' potential in their domination plans. Using their incredible scientific knowledge, they ingrafted the ability to swim into all the ground-dwelling mutants' gene pools, thereby allowing them access to the mainland where they could wreak havoc.