Metamorphosis mechanical beast living in the Great Lakes

Synopsis

The strange monster of Lake Yamanaka turns out to be the Mechanical Beast Granada E3, created by Dr. Hell. Unaware of the truth, Shiro and his friends set out across the lake in a boat, determined to take the creature alive, and end up swallowed by Granada E3. Granada E3's insides were a nuclear reactor — it had been eating uranium ore exposed on the lakebed, building up plutonium, and converting it into nuclear energy. That energy had already reached a level capable of destroying Mazinger Z.