China is getting serious about defending against our submarine force:
It was recently revealed that China began installing underwater passive sonar systems in its coastal waters back in 2011. This enables China to monitor submarines operating off its coasts and, presumably, in the South China Sea. South Korea did the same in 2011 when it announced that it was installing underwater submarine sensors off its coasts and this was apparently completed in 2013. The South Korean effort was in response to North Korea using a small submarine to torpedo a South Korea patrol ship in 2010. China simply wants to keep foreign warships as far awau as possible, even if it means trying to force them out of international waters.
I wonder how good their listening network really is, and how well trained their operators are. The network wouldn’t do any good protecting their commercial shipping, which could be sunk with ease (or stopped with greater risk and effort) anywhere on the high seas advantageous to the US Navy. But it could be of great use for keeping our boats out of their coastal waters, where we like to sneak in and spy.