On June 11, the American destroyer USS John McCain, while training off Subic Bay in the Philippines, had its towed sonar array collide with a Chinese submarine. The U.S. Navy did not reveal if the American ship had detected the sub before the collision. If the array was not activated, its sound (sonar) detectors would not have detected the sub. The Chinese admitted the sub was one of theirs, and the boat was apparently following the American ship unaware that a sonar array (which usually operates over a hundred meters beneath the surface, and two kilometers behind the ship towing it) was there.
The Chinese sub was probably a diesel-electric sub, which is a lot quieter under water than one of their nuclear powered models. The incident brings up memories of similar incidents with Russian subs during the Cold War. Some of these collisions were believed to be intelligence operations, an effort to grab portions of the American sonar array for examination (and reverse engineering.)
U.S. anti-submarine forces (subs, aircraft and surface ships) are increasingly playing tag with Chinese subs. As was done with Russian subs during the Cold War, the American sailors want to hone their skills at finding Chinese subs. All this effort is kept quite secret, as any information about American successes or failures, can be useful to the Chinese.