Source: http://nationalinterest.org/feature/are-...

In a piece for TNI, the report’s author, Bryan Clark, lays out the problem in more layman's terms:

Since the Cold War submarines, particularly quiet American ones, have been considered largely immune to adversary A2/AD capabilities. But the ability of submarines to hide through quieting alone will decrease as each successive decibel of noise reduction becomes more expensive and as new detection methods mature that rely on phenomena other than sounds emanating from a submarine. These techniques include lower frequency active sonar and non-acoustic methods that detect submarine wakes or (at short ranges) bounce laser or light-emitting diode (LED) light off a submarine hull. The physics behind most of these alternative techniques has been known for decades, but was not exploited because computer processors were too slow to run the detailed models needed to see small changes in the environment caused by a quiet submarine. Today, “big data” processing enables advanced navies to run sophisticated oceanographic models in real time to exploit these detection techniques. As they become more prevalent, they could make some coastal areas too hazardous for manned submarines.