Dive to the Deep Six

In the bathyscaphe named Quetzalcoatl Mike Slater and his diving partner Manfred Richter hear pings from more than one source. They are retrieving a tube from a sunken submarine and have experienced a desperate ordeal before hearing the sonar sounds that are both mysterious and unsettling. All they want to do is return to the surface and exit the steel sphere that has taken them down to the bottom of the Atlantic Ocean.