Chinese submarines cruise South China Sea

A submarine of a submarine flotilla of the South China Sea Fleet under the Navy of the Chinese People's Liberation Army (PLA) is cruising in the South China Sea on a sunny day in mid-January 2013. The depth sensor finally read 200 meters. Luo Zuyun, captain of the submarine, said that in this depth we could eliminate the threat from satellite-sounding and the natural noises of the sea with a positive sound velocity gradient could drown out those of the submarine, therefore, the submarine could remain undiscovered in the sea.