November 8, 2026: The U.S. Navy is building a new submarine, the Virginia-class USS Baltimore, which is equipped with multiple seabed sensors to map and explore seabed topography and composition. This specialized sub is to replace the Seawolf-class USS Jimmy Carter as the U.S. Navy's specialized submarine. Earlier this year, to make American submarines more effective, the U.S. Navy is developing Seabed Superiority. This is intended for current operations in the Baltic Sea and the North Atlantic trade routes. This depends on a seabed sensor network, something like the Cold War-era SOSUS.

Currently, China operates 64 well-equipped oceanographic research vessels, most built in the past 15 years. This modern fleet is larger than the combined fleets of the U.S. Academic Research Fleet, the National Oceanic and Atmospheric Administration’s research fleet, and the U.S. Navy’s oceanographic survey vessels. China’s fleet includes polar-capable ships, fisheries research vessels, and ships designed to deploy autonomous unmanned vehicles, remotely operated vehicles, or manned submersibles. 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 XLUUVs (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, travel thousands of miles, and periodically transmit their locations and observations of temperature, salinity, and depth via satellite. The US has long used its equivalents, 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 bearing line to a sound source, as well as magnetometers for submarine detection.

Along the First Island Chain, China’s cabled seafloor observatory, the National Seafloor Scientific Observation Network, serves dual purposes: 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 includes acoustic arrays, seismometers, physical and chemical sensors, and navigation beacons that enable docking and acoustic navigation for underwater drones. Because acoustic arrays and gliders can serve as tripwires 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 to initialize them more accurately. In 2025, China achieved a 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 U.S. models, which simulate global conditions at only 4-km to 9-km resolution. LICOM K++ can simulate fine-scale processes such as oceanic internal waves and microscale eddies—processes that operational U.S. global models must approximate mathematically.

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 match U.S. naval capabilities in the western Pacific.

Thirteen years earlier, China began installing passive underwater sonar systems along its coast. This enabled China to monitor submarines operating off its coasts and, presumably, in the South China Sea. South Korea did the same, announcing it was installing underwater submarine sensors off its coast and apparently completing the effort in 2013. The South Korean effort was in response to North Korea using a small submarine to torpedo a South Korean 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.

China and South Korea did not reveal technical details, but this sort of system resembles the passive sonar system the United States deployed on the seabed in key areas during the Cold War. SOSUS (Sound Surveillance System) consisted of several different networks. The CAESAR network covered the continental shelf areas bordering the North Atlantic. In the North Pacific, COLOSSUS operated, plus a few sensors in the Indian Ocean and a few other places no one would talk about. Underwater passive sonars listened to everything and sent data via cable to land stations. From there, it went 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 contact quality, the circle could shrink to as little as ten kilometers. The major drawback was that it did not cover deep-water areas beyond 500 kilometers from the continental shelf edge. This is not a problem for the South Korean or Chinese systems, as both cover only coastal waters or shallow offshore areas, such as the South China Sea.

SOSUS systems are very expensive to maintain. SOSUS survived the end of the Cold War by making its sensors available for civilian research and adopting cheaper, more powerful electronic and communications technology. Although the U.S. has shut down many parts of SOSUS, it has put additional portable SOSUS gear into service and deployed it as needed.

South Korea benefited from U.S. help with SOSUS and in collecting and processing submarine sound signatures in the area. The U.S. also helped South Korea obtain more sensitive passive sonar systems that can locate submarines more accurately. The U.S. has been conducting research in this area and knows such cooperation would give the U.S. access to South Korea's SOSUS. South Korea also has the design and manufacturing capability for this sort of device. South Korea placed the first SOSUS system off its west coast, near the North Korean border. North Korean submarines that travel underwater, run on batteries, and operate 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.

If the U.S. were at war with China, one of its main goals would be to disrupt Chinese shipping with submarines, especially the quiet, heavily armed Virginia-class nuclear-powered submarines operating in the Pacific. As China became a potential threat, the Pacific Fleet received more Virginia-class submarines. The Virginias were there to shut down Chinese trade on short notice by deploying hundreds or thousands of naval mines near Chinese ports. Their primary weapon will be naval mines. China’s mine-clearing capability is believed to be limited.

James F Dunnigan