Varan: The Unbelievable (US Release 1962)
If you're into Japanese rubber suit monster movies here's one for you!
In an effort to find an economic means of purifying salt water, a joint U.S.-Japanese military command is set up on an isolated Japanese island where an unusual salt water lake is situated. However, their purifying experiments arouse the prehistoric monster Obaki from hibernation at the lake's bottom, and it proceeds to attack Japan.
Commander Jim Bradley of the United States Navy is sent to Japan to test a chemical, which if successful, will be used to desalinate water. Unfortunately, the indigenous population are against the experiments in their lake, which happens to have the right amount of salt content, because it is feared that the chemicals will re-awaken their god, Obake. After Bradley's wife Anna intervenes, the populace is evacuated and the experiment goes on as scheduled. Unfortunately, as predicted, the chemicals reawaken their god, which happens to be a giant prehistoric beast. After destroying the local village, the creature then sets his sites on the city of Oneida.
Varan is later a monster that lives on monster island with godzilla!