Latitude Zero.

A massive underwater volcano erupts and puts a group of investigative scientists in danger. They are rescued by an atomic super submarine named The Alpha under the command of Captain McKenzie. The group is quickly taken to a vast underwater city known as Latitude Zero, a fantastic, Atlantean type utopia, a world beneath the ocean with its own sun. It is soon discovered that Captain McKenzie is at war with the evil Dr. Malic, a cruel scientist who wishes to rule mankind all the while conducting genetic experiments on humans and animals. Malic sends his agents to kidnap Dr. Okada, a human scientist who has created a serum that can immunize exposure to radiation.