The submarine of 1939 had an awkward weakness: sooner or later, it had to surface. Beneath the water it ran on batteries. To recharge them, its diesel engines needed air. A vessel built to disappear eventually had to reveal where it was.

Ross Gunn had been looking for a way around that problem when he attended a DC briefing in March of 1939. The speaker was Enrico Fermi, who had reached New York barely two months earlier. Fermi and his wife, Laura, had used his trip to Stockholm to accept the Nobel Prize as their escape from Mussolini’s Italy.

Fermi described the new science of splitting uranium. No reactor had yet been built, and a controlled chain reaction remained theoretical. He believed one might be possible if physicists could solve problems of chemical purity. The discussion naturally turned toward a bomb. Gunn followed the same physics in another direction. If fission could produce heat, heat could make steam, and steam could drive a submarine’s turbines without oxygen from above.

Within days, Gunn and Naval Research Laboratory Director Hollis Cooley took the idea to Rear Admiral Harold Bowen. Gunn later remembered how strange it felt to ask a senior officer to support such a “fantastic program”. Bowen approved $1,500, the first federal money spent on nuclear-fission research. The Navy had entered the field seven months before the Army effort that became the Manhattan Project.

The grant produced no sudden breakthrough. By year’s end, Gunn’s team had made only a few grams of uranium hexafluoride. The Navy’s thermal-diffusion method, advanced by physicist Philip Abelson, was later absorbed into the Manhattan Project. The bomb arrived first.

After the war, Gunn and Abelson returned to the older idea. Hyman Rickover later drove the reactor program that turned it into a vessel. On January 17, 1955, sixteen years after Fermi’s lecture, USS Nautilus left its pier. Its radio message contained four words: “UNDERWAY ON NUCLEAR POWER.”