It's official now.
Japan, U.S. run undersea detection system for Chinese submarines
TOKYO — Japan’s Maritime Self-Defense Force and the U.S. Navy are jointly operating an undersea submarine detection system on the floor of the Pacific side of the Nansei Islands chain including Okinawa as part of efforts to counter China’s rising maritime activities, Kyodo News learned Wednesday.
The latest model of the sound surveillance system, or SOSUS, allows Tokyo and Washington to detect Chinese submarines traveling from the East China Sea and Yellow Sea to the Pacific, according to sources in the Defense Ministry and the MSDF.
It is the first time that the existence of a listening system solely targeting China has been revealed.
As the system is “top secret in the Japan-U.S. security system,” the MSDF explains outlines of the U.S.-developed system only to a group of key figures in the Japanese government including the prime minister and the defense minister..... to read more
I guess everyone speculated it's existence including myself.