The Champions 1969 S01E20

Episode: The Silent Enemy. The entire crew of the submarine 'Keppel' are found dead at their posts. All have died of cardiac arrest. The Champions join a fresh crew to re-trace the voyage but in the Atlantic they lose radio control. They find an uncharted island which is being used for illegal nuclear tests by a country whom the U.N. denied nuclear weapons. The 'Keppel' crew were killed because they came across it.