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China has, of course, denied the incident ever occurred, but some 55 Chinese sailors aboard a submarine designated 093-417 are presumed dead after an accident in the Yellow Sea, off of China’s Shandong Province. British intelligence reports the submariners died after a “catastrophic” failure of the vessel’s oxygen systems on August 21, 2023.

“Our understanding is death caused by hypoxia due to a system fault on the submarine. The submarine hit a chain and anchor obstacle used by the Chinese PLA Navy to trap US and allied submarines,” the report read. “This resulted in systems failures that took six hours to repair and surface the vessel. The onboard oxygen system poisoned the crew after a catastrophic failure.”

Since the report is based on intelligence, the United Kingdom has declined to comment on the veracity of the reports, while Beijing maintains the incident is a rumor. If the submarine really did run into its own chain and anchor trap, and its batteries were low, then it’s likely the air purifiers and air treatment systems would eventually fail.

While U.S. and UK submarines have devices that can turn carbon dioxide into oxygen, allowing them to remain submerged for long periods of time, not every country has access to the same technology. The “chain and anchor” is a device intended to trap and destroy enemy vessels under the sea. The idea is simple: a heavy chain of metal balls and other objects (that can be up to several miles long) is attached to two anchors deployed on the seafloor.

The size and weight of the chain can cause heavy damage to a vessel’s rudder and propeller, delaying the submarine for hours or even days, which as we can see from the PLA Navy’s alleged incident, can be deadly for the ship and its crew. The submarine may even be dragged to the seafloor with the trap.

These kinds of chain and anchor traps are used to protect strategic or sensitive positions, such as harbors, ports or shipping lanes. The document from British intelligence believes this particular trap was intended to ensnare American or British spy submarines operating off the Chinese coast. Shandong is a Province home to many cities with populations above 9 million, roughly the size of New York City. Qingdao, with a population of 10 million, is the largest city in the province.

While much of the world’s focus has been on Chinese naval operations in the South China Sea, especially the waterways between Taiwan and mainland China, the Yellow Sea is also a strategically important body of water along the Chinese coast. The country shares maritime borders with North Korea, South Korea, and Japan in these waters and it’s an important site for commercial fishing, oil and gas exploration and commercial trade. The Yellow Sea is also an important supply route between China and Russia, who supplies the People’s Republic with nuclear material used in creating nuclear warheads.

The loss of 093-417 is a big deal for the PLA Navy. It’s one of the more advanced submarines in its fleet, as it runs quieter and carries more advanced weaponry (including anti-ship missiles and new, advanced torpedoes).