China says it picked up a submarine detector dropped by a US Navy aircraft in the South China Sea

• Chinese state media shared a video of what it characterized as a submarine detection device dropped by a US Navy aircraft.
• The apparent sonobuoy was discovered in the South China Sea near the Second Thomas Shoal. 
• Military maritime patrol aircraft, including China's planes, routinely use systems like these.

China's coast guard found a submarine detection device dropped by a US Navy aircraft somewhere in the South China Sea, a state broadcaster reported.

Many militaries, including China, use sonobuoys and acoustic sensors. Maritime patrol aircraft often use them and other detectors in open waters to track submarine movements and locations.

Video footage shared by Chinese state media on Wednesday showed a US Navy aircraft, what appears to be a P-8 Poseidon, dropping an object resembling a sonobuoy into the South China Sea.

Business Insider was unable to independently verify the reported details of the video posted to Chinese social media. The footage showed a close-up examination of the object, which Yuyuan Tiantan, part of the China Media Group, identified as a "submarine detector" and reported was recovered by the Chinese coast guard.

The video identifies the object's manufacturer as Ultra Electronics Undersea Sensor Systems, Inc. It's a British company which makes sonobuoys and underwater sensors.

The US Department of Defense didn't respond to Business Insider's request for comment.

Chinese state media said its coast guard picked up the device near the Second Thomas Shoal in the South China Sea, contested territory claimed by both China and the Philippines.

The two South China Sea claimants have clashed in the area, increasingly over the past few years and especially in recent months, with Chinese Coast Guard ships firing water cannons at Philippine supply boats and ramming Philippine vessels.

Chinese coast guard personnel recently disrupted a supply run with bladed weapons, drawing complaints of piracy from Manila.

The Second Thomas Shoal, which China calls Ren'ai Jiao, is located far from the Chinese mainland, as well as the Yulin Naval Base on Hainan Island, where China houses its submarines, though its boats operate across the strategic South China Sea.

Sonobuoys and other similar sensors are small, common instruments in anti-submarine warfare and underwater acoustic research. They have long been used by militaries for monitoring submarine locations and movements, as well as non-military purposes.

There are various kinds of sonobuoys that deliver active and passive collection of data or serve specific operations. In the state media video, China said that the "probe" was being used to detect subs and counter sub signals.

Like the US Navy, China has its own anti-submarine warfare and maritime surveillance aircraft, such as the KQ-200 that are capable of deploying sonobuoys like the SQ-4s and SQ-5s.

In response to the recent discovery by the Chinese coast guard, Ministry of National Defense spokesperson Wu Qian said that China firmly opposes US warships and aircraft stirring up trouble in the South China Sea, per Chinese media reporting.

US military ships and aircraft routinely operate in the international waters of the South China Sea unilaterally and with allies and partner nations in support of what it calls a "free-and-open Indo-Pacific," flying and sailing wherever international law allows.