Gamera: The Invincible

In the Arctic, an unknown aircraft is shot down by an American jet fighter. The aircraft crashes and its cargo, a low-level atomic bomb, explodes. The resulting cataclysm awakens a prehistoric monster that has the appearance of a giant turtle with teeth and large tusks. Japanese scientists on an expedition nearby (including Drs. Hidaka, Kyoko and Aoyagi) are given a "devil stone" by an Inuit chieftain, who explains that the creature is called Gamera.