China will be building an undersea monitoring system in South China Sea.

http://edition.cnn.com/2017/05/29/asia/south-china-sea-underwater-observation-system/

This is akin to USA "SOSUS".

https://en.wikipedia.org/wiki/SOSUS

China will be able to know the undersea activities soon with this system.  It will be harder for USA submarines and undersea drones to be undetected.