On the morning of January 17, 1955, the Thames River at Groton looked much as it always had in winter. Gray water, cold air, men in heavy coats moving with the practiced economy of sailors who knew their business. Nothing in the scene warned the casual observer that the age of naval propulsion was about to change course. At 11:00 a.m., the submarine tied to the pier eased herself free, not with drama or spectacle, but with a kind of quiet confidence. A few minutes later, a short message blinked out by signal lamp to the tender alongside. “Underway on Nuclear Power.” It was ten words, plain and unsentimental, and it marked the first time in human history that a vehicle moved under the control of sustained nuclear fission. The boat was the USS Nautilus, and the world did not yet grasp what had just slipped its moorings.