The U.S. Navy’s dominance under the waves is threatened by intertwined developments in sensor technology: smaller, better sensors and improved data processing to interpret sensor inputs. Because these developments are available to potential adversaries, they will erode the Navy’s advantage in submarine quieting, eventually threatening these key platforms and making vital warfighting and intelligence-gathering tools vulnerable. These technologies especially threaten the nuclear-powered attack submarines (SSNs) that operate close to potential adversaries’ coasts. The growing risks demand continual improvement in concealment of U.S. submarines and detection of potentially hostile ones.