April 15, 2026: China is using 64 well-equipped oceanographic research vessels, the majority of which were built within the past 15 years. This modern fleet is larger than the U.S. Academic Research Fleet, the National Oceanic and Atmospheric Administration’s research fleet, and the U.S. Navy’s fleet of oceanographic survey vessels combined. China’s fleet includes polar-capable ships, fisheries research vessels, and ships designed for autonomous unmanned vehicle, remotely operated vehicle, or manned submersible deployments. A technological showpiece is the Zhu Hai Yun, which operates as a mothership for more than 50 unmanned vehicles, including aerial drones, autonomous surface vehicles, and underwater gliders. Its unmanned vessels can operate simultaneously, providing persistent surveillance of an area extending 160 kilometers across, 4 km above the surface, and 1.5 km below the surface while the mothership remains underway.

China has developed five XLUUV\Extra-Large Unmanned Underwater vehicles measuring 15 to 20 meters long. These vehicles can sense physical data, map the seafloor, and carry torpedoes or mines. The XXLUUVs are large enough to carry towed-array sonars and voyage across the Pacific.

For persistent surveillance throughout the water column, China’s Haiyan and Haiyi gliders can operate for months, traveling thousands of miles and periodically transmitting their locations and observations of temperature, salinity, and depth via satellite. The US has long used its equivalents of these, some of which China has obtained because so many have washed up on stray Pacific beaches after accidents. A military variant of the Haiyan glider is equipped with vector acoustic sensors that can determine a line of bearing to a sound source, as well as magnetometers for submarine detection.

On the seafloor along the First Island Chain, China’s cabled seafloor observatory, the National Seafloor Scientific Observation Network, serves the dual purposes of environmental research and acoustic monitoring of marine traffic. The East China Sea segment is focused on the shallow continental shelf, while the South China Sea segment reaches depths of 3,000 meters. The network consists of acoustic arrays, seismometers, physical and chemical sensors, and navigation beacons that provide docking and acoustic navigation for underwater drones. Because the acoustic arrays and gliders can be a tripwire for submarine detection, the network is viewed as an underwater Great Wall of China.

To improve ocean and acoustic forecasting, global and regional ocean models assimilate data from ships, satellites, unmanned vehicles, and moored sensors for more accurate model initialization. In 2025, China achieved a major breakthrough in ocean modeling with the LICOM K++ model, which provides 1-km horizontal resolution for three-dimensional, global ocean simulations. This is significantly better than the US models which simulate global conditions with only 4-km to 9-km resolution. LICOM K++ can simulate fine-scale processes such as oceanic internal waves and microscale eddies—processes that must be mathematically approximated by operational U.S. global models.

With sustained funding and a blurred line between civilian research and military applications, China has closed the gap in a domain in which the United States has long held a significant advantage: stealthy naval operations. By the end of its current Five-Year Plan, China may achieve parity with U.S. naval capabilities in the western Pacific.

Thirteen years ago, China began installing underwater passive sonar systems in its coastal. This enabled China to monitor submarines operating off its coasts and, presumably, in the South China Sea. South Korea did the same when it announced that it was installing underwater submarine sensors off its coasts and this was apparently completed in 2013. The South Korean effort was in response to North Korea using a small submarine to torpedo a South Korea patrol ship in 2010. China simply wants to keep foreign warships as far away as possible, even if it means trying to force them out of international waters.

Technical details were not revealed by China or South Korea, but this sort of thing is similar to the system of passive they just listen to sonars the United States deployed on the sea bottom in key areas during the Cold War. SOSUS\SOund SUrveillance System consisted of several different networks. On the continental shelf areas bordering the North Atlantic was the CAESAR network. In the North Pacific there was COLOSSUS plus a few sensors in the Indian Ocean and a few other places that no one would talk about. The underwater passive sonars listened to everything and sent their data via cable to land stations. From there it was sent back to a central processing facility, often via satellite link. SOSUS was accurate enough to locate a submarine within a circle no wider than 100 kilometers. That's a large area, but depending on the quality of the contact, the circle might be reduced up to ten kilometers. The major drawback of the system was that it did not cover deep water areas more than 500 kilometers from the edge of the continental shelf. This is not a problem for the South Korean or Chinese systems, as both only cover coastal waters or shallow offshore areas like the South China Sea.

SOSUS systems are very expensive to maintain. SOSUS managed to survive the end of the Cold War by making its sensors available for civilian research and by using cheaper and more powerful electronic and communications technology. While many parts of the SOSUS have been shut down, additional portable SOSUS gear has been put in service, to be deployed as needed.

South Korea had the advantage of being able to get help from the United States about SOSUS and how to collect and process the sound signatures of submarines operating in the area. The U.S. was also able to help South Korea obtain more sensitive passive sonar systems that can identify submarine location more accurately. The U.S. has been doing research in this area and knows that such cooperation would result in American access to the South Korean SOSUS. South Korea also has the design and manufacturing capability for this sort of device. The first South Korea SOSUS system was placed off the west coast, near the North Korean border. North Korean submarines, travelling underwater, using battery power and near the coast, are very hard to detect. The South Korean SOSUS will help even the odds. China’s Internet based espionage efforts have probably already stolen a lot of American SOSUS secrets and that helped a lot.