Octopus

Low-budget creature feature. When an unknown assailant attacks the US submarine Roosevelt and drags the vessel to the bottom of the sea, the crew members are horrified to see dozens of wrecks on the ocean floor, including a Soviet sub that was lost in 1962 while on its way to Havana during the Cuban missile crisis.

Whatever happened can only be the work of a creature of unbelievable size and strength, but what is it and how can it be stopped?