China is conducting a massive underwater mapping and monitoring operation in the Pacific, Indian and Arctic oceans, amassing detailed knowledge of sea conditions that naval experts say is crucial for waging undersea warfare against the United States and its allies.
The Dong Fang Hong 3, a research vessel operated by the Ocean University of China, spent 2024 and 2025 sailing back and forth in the seas near Taiwan and the US bastion of Guam, as well as around strategic stretches of the Indian Ocean, ship tracking data shows. In October 2024, it tested a suite of powerful Chinese ocean sensors capable of identifying underwater targets near Japan, and visited the same area again last May. And in March 2025, the vessel sailed through the waters between Sri Lanka and Indonesia, covering the approaches to the Strait of Malacca, a critical point for maritime trade.
The ship has been conducting mud surveys and climate research. But a paper co-authored with academics from the Ocean University of China shows that it has also conducted extensive mapping of deep-sea conditions. Naval operations experts and U.S. Navy officials say the kind of deep-sea data the Dong Fang Hong 3 collects by mapping and placing sensors in the ocean gives China the insight into underwater conditions it would need to more effectively deploy its submarines and hunt down those of its adversaries.
The Dong Fang Hong 3 is not operating alone. It is part of a larger ocean mapping and monitoring operation involving dozens of research vessels and hundreds of sensors. Although the survey is for civilian purposes, some of the surveys cover fishing grounds or areas where China has mineral exploration contracts. It also serves military purposes.
To gather information about the underwater terrain, the research vessels map the seabed as they move back and forth in tight formations. Tracking data shows this type of movement from the vessels tracked by Reuters across large swaths of the Pacific, Indian and Arctic oceans. At least eight of the vessels have been mapping the seabed, while another 10 have been carrying equipment used for mapping.
The data from the ships’ surveys “would be potentially invaluable in preparing the battlespace” for Chinese submarines, said Peter Scott, a former chief of Australia’s submarine force. Any good military submariner will go to great lengths to understand the environment in which they operate, he explained.
Ship tracking data shows that China's seabed exploration efforts are focused in part on militarily important waters around the Philippines, near Guam and Hawaii, and near U.S. military installations on Wake Atoll in the North Pacific.
"The scale of what they are doing is more than just resources," said Jennifer Parker, an associate professor of defence and security at the University of Western Australia and a former Australian anti-submarine warfare officer. If you look at the scope itself, it’s very clear that they intend to have an expeditionary naval capability in swampy waters that is also built around underwater operations, he said.
Even when the data is collected for scientific purposes, the integration of civilian research and military-technological development has become a key focus of the Chinese government under President Xi Jinping, according to Parker and other experts. Beijing calls this approach “civil-military fusion.”
In testimony before a congressional committee this month, Rear Admiral Mike Brooks, commander of the U.S. Office of Naval Intelligence, said China has dramatically expanded its geodetic efforts, providing data that allows submarines to navigate, conceal, and position sensors or weapons on the seabed. He said the potential collection of military intelligence “is a major challenge for the Chinese government.” by Chinese research vessels is a strategic concern.
The United States has recently re-examined its own ocean mapping and surveillance efforts, but it typically does so with military vessels that are allowed to turn off the tracking system monitored by civilian software. Chinese civilian research vessels also sometimes turn off tracking.
This is the first time the scale of China’s mapping and surveillance efforts in the Pacific, Indian and Arctic oceans has been reported. Previous reports have revealed some of the efforts around Guam and Taiwan, as well as in parts of the Indian Ocean. "Frankly, it’s mind-boggling to see the sheer scale of China’s maritime research," said Ryan Martinson, an associate professor specializing in Chinese maritime strategy at the U.S. Naval War College. For decades, the U.S. Navy has been able to take an asymmetric advantage in its knowledge of the ocean battlespace, Martinson added. China’s efforts threaten to undermine that advantage. This is clearly deeply troubling, he said.
The data that Chinese research vessels collect about the seabed and water conditions is crucial for undersea operations and anti-submarine warfare, according to naval experts. Most obviously, Australian defense scientist Parker said, commanders need information about underwater terrain to avoid collisions and hide their ships.
But this data is also essential for detecting submarines operating a few hundred meters below the surface. Submarines are usually identified by the sounds they make or by the echoes of signals sent by sonar systems. Tom Shugart, a former U.S. submarine commander who is now a senior associate professor at the Center for a New American Security, said the movement of these sound waves changes depending on the underwater landscape. Sound waves and submarine movements are also affected by water temperature, salinity and currents.
The vessels involved belong to Chinese state organizations such as the Ministry of Natural Resources or state research institutions such as Ocean University, whose president in 2021 publicly celebrated the "close ties" of the university with the Chinese navy and its commitment to "building maritime power and national defense".
China has conducted its most comprehensive ocean survey to the east of the Philippines, which lies along the First Island Chain, a chain of territories largely controlled by America's allies that stretches from the Japanese islands in the north through Taiwan to Borneo in the south. The chain forms a natural barrier between China's coastal seas and the Pacific Ocean.
"They are paranoid about being locked into the First Island Chain," said Peter Leavy, a former Australian naval attaché to the United States and now president of the Australian Naval Institute. China's mapping "shows a desire to understand the maritime domain so they can break through". Tracking data shows that China’s mapping also extends to the waters around Guam, home to some US nuclear submarines.
Strikingly, Chinese ships have also mapped the waters around Hawaii, one of America’s other regional military hubs. They have explored an underwater ridge north of a naval base in Papua New Guinea that the US recently gained access to, and have been scouting around Christmas Island, an Australian territory on a route between the South China Sea and a vital Australian submarine base.
China’s efforts extend further afield. China has mapped large parts of the Indian Ocean, a critical route for Chinese imports of oil and other resources from the Middle East and Africa. "China has some key vulnerabilities when it comes to maritime trade dependencies," said Parker, the former anti-submarine warfare officer.
Chinese ships have also mapped the seabed west and north of Alaska, a key sea route to the Arctic. Beijing has identified the Arctic as a strategic frontier and has declared its ambition to become a polar great power by the 2030s. Beijing's extensive exploration and growing underwater capabilities are "symptomatic of China's rise as a leading maritime power," said Shugart, the former submarine commander.
Around 2014, Wu Lixin, a scientist at Ocean University, proposed an ambitious effort to create a "transparent ocean" by deploying sensors that would give China a comprehensive view of the state of the water and its movement through certain areas. The proposal quickly received support of at least $85 million from the Shandong provincial government, according to comments from Shandong officials.
The project began in the South China Sea, where public statements from the Ocean University boast that it has already built a monitoring system covering the deep-sea basin.
Brooks, director of the U.S. Office of Naval Intelligence, told a congressional committee that China is building underwater surveillance networks that "collect hydrographic data - water temperature, salinity, currents - to optimize sonar performance and allow constant surveillance of submarines passing through critical waterways like the South China Sea."
After studying the South China Sea, Chinese scientists have expanded the Transparent Ocean Project to the Pacific and Indian Oceans. In the Pacific Ocean, data from China’s Ministry of Natural Resources, Ocean University and the Shandong government show that China has deployed hundreds of sensors, buoys and underwater systems to detect changes in water conditions such as temperature, salinity and underwater movement across the ocean east of Japan, east of the Philippines and around Guam.
In the Indian Ocean, documents from the Chinese Academy of Sciences and the Ministry of Natural Resources describe a sensor network surrounding India and Sri Lanka, including along an underwater mountain range known as the Ninety East Ridge. The ridge – which Chinese ships have also searched, according to Starboard data – is one of the longest underwater mountain ranges in the world and is located on the approach to the strategically important Strait of Malacca, through which much of China’s oil supplies pass. The Ocean University and the Institute of Oceanography, part of the Chinese Academy of Sciences, said the expanded network of sensors now provides China with real-time data on water conditions and underwater movements.
Some naval warfare experts have expressed caution about that claim, given the technical challenges of communicating real-time data from underwater. But even the delayed data is valuable, Parker said, because it could help China detect U.S. submarine operations.
Many of the sensors are located in sensitive locations. For example, Reuters recently reported on U.S. efforts to fortify a key strait between Taiwan and the Philippines to cut off China’s access to the Pacific Ocean. Research by the Ocean University shows that China has deployed advanced sensors in parts of the strait that U.S. submarines would navigate to reach the South China Sea.
Chinese scientists say the sensors monitor changes in climate and ocean conditions. But in 2017, government officials from Shandong province said the Transparent Ocean project was designed to “ensure maritime defense and security” and explicitly compared the project to the U.S. military’s efforts to build a U.S. network of ocean sensors. Wu, the founder of the mapping program, now runs the network through the National Laboratory of Marine Science and Technology in Qingdao, whose partners include the China Naval Academy of Submarine Warfare, according to the academy’s website.
China’s mapping and monitoring capabilities give it advanced tools to detect enemy submarines and deploy its own in some of the world’s most contested waters. "This is a demonstration of China's far-reaching reach," said Colin Koh, a senior fellow in maritime security at the Singapore Institute of Defence and Strategic Studies (RSIS). "They already have a fairly good idea of the maritime domain they hope to operate in, both in peacetime and in wartime."
Chinese researchers similarly see strategic value in their work. Zhou Chun, a researcher at the Ocean University who leads sensor arrays in the Indian and Pacific Oceans. He noted that his work has shown him the rapid development of China's maritime defense and military capabilities. In the future, Zhou promised to "transform cutting-edge scientific and technological advances into new types of combat capabilities for our military at sea."