The United States has filed a formal diplomatic protest over China's inexplicable seizure on Dec. 15 of an "underwater glider" being operated by the U.S. Navy in international waters of the South China Sea off the Philippines.

China has so far refused to respond or explain why a naval vessel of the People's Liberation Army Navy (PLAN) shadowing the USNS Bowditch (T-AGS 62), a Pathfinder-class oceanographic survey ship, seized the glider about to be retrieved by the U.S. ship.

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The Bowditch was designed for oceanographic sampling and data collection and the handling, monitoring and servicing of remotely operated vehicles (ROVs) such as underwater or ocean gliders, which are a class of unmanned underwater vessels (UUVs).

The U.S. Navy said the seizure of the glider took place in international waters some 50 nautical miles northwest of Subic Bay in the Philippines. It said there is no precedent in recent memory for an act of high seas piracy such as this.

It said the glider wasn't a military glider but an unclassified UUV conducting "routine operations in accordance with international law." The gliders measure events such as salinity and temperature.

The Bowditch was retrieving one vehicle when a Chinese warship arrived and stole the second UUV using a small boat. The U.S. sent radio messages requesting the drone be returned but the Chinese warship ignored the messages it confirmed receiving.

"We call upon China to return our UUV immediately, and to comply with all of its obligations under international law," said the Pentagon in a statement.

Hawkish Republicans demanded retaliation for the Chinese piracy. Senate Armed Services Committee Chairman John McCain said this wanton deed requires a strengthened US military posture in the South China Sea to deter future aggression.

"This brazen provocation fits a pattern of increasingly destabilizing Chinese behavior ... and this behavior will continue until it is met with a strong and determined US response," said Sen. McCain stated.

Some political analysts, however, surmise China might be sending a message to President-elect Donald Trump over their displeasure about Trump's dismissal of the One China Policy and his buddying up to Taiwan.

"Knowing Chinese military officials for many years and how orders are communicated from the highest power centers in Beijing down to commanders on the ground or water, this was very likely a highly planned and escalatory move to show China will not take matters lightly when it comes to President-elect Trump's phone call and comments on Taiwan, or Chinese actions overall," said Harry Kazianis, director of defense studies at the Center for the National Interest.

Another reason for this act of piracy is China's intention to quickly advance its own ocean glider program that lags far behind the U.S. China will reverse engineer the unidentified glider it stole from the U.S. Navy.

China has continued developing deep-diving underwater gliders despite a decades-old U.S. ban on exports of these vehicles to China out of concern China will put the torpedo-shaped gliders to military use.

Last August, Chinese media revealed a Chinese underwater glider named Haiyi-7000 that plunged 5,751 meters into the Mariana Trench, the world's deepest ocean trench, had aroused the interest of the People's Liberation Army (PLA), China's armed forces.

That depth was about 250 meters shy of the world record for underwater gliding of 6,000 meters set in 2009 by a Seaglider developed by researchers at the University of Washington. Another UW Seaglider also holds the world's endurance record, operating for nine months and five days in the Pacific Ocean.

An underwater glider is an autonomous underwater vehicle (AUV) that uses small changes in its buoyancy to convert vertical motion to horizontal motion, thereby propelling itself forward with very low power consumption.

Underwater gliders, which are motorless and have wings, trace an up-and-down sawtooth motion through the water. This method of propulsion allows gliders to travel farther than AUVs powered by electric motor-driven propellers. The United States is the world leader in this technology while the U.S. Navy is one of its biggest backers.

A huge advantage of military underwater gliders is they can cruise noiselessly and autonomously beneath the surface without human intervention for days, months or even an entire year. All the while, the gliders keeps collecting military data or monitoring activity over a vast ocean area.

An underwater glider armed with an explosive warhead can also be programmed to attack underwater targets such as enemy submarines. But, like wolves, armed gliders can be unleashed in packs of up to 100 vehicles to deny enemy submarines access to specific areas.

The Haiyi-7000 is one of a number developed by a team from the Shenyang Institute of Automation, Chinese Academy of Sciences led by Prof. Yu Jiancheng.

It was launched from Tansuo-1, the academy's new 94 meter research ship, which is part of the government's rapidly expanding scientific research fleet.

Tagsunderwater glider, United States, Piracy, china, USS Bowditch, unmanned underwater vessels, UUV, John McCain