The next undersea arms race is not only in submarines. It is in seabed knowledge, acoustic advantage, and control of the battlespace before the shooting starts.
China is not only building more naval power.
It is building more knowledge of where that naval power will fight.
That is the strategic shift now becoming visible. Reuters reported this week that China is conducting a vast undersea mapping and monitoring effort across the Pacific, Indian, and Arctic oceans, using dozens of research vessels and hundreds of sensors to collect data in waters near Taiwan, Guam, Japan, the Philippines, Hawaii, the Malacca Strait, and the Arctic.[1]
That is not routine marine science.
It is battlespace preparation.
The side that understands the seabed, the sound layers, the currents, the choke points, and the hiding places first does not simply gain better charts. It gains a quieter submarine force, a stronger anti submarine picture, and a much better chance of shaping undersea warfare before war begins. Reuters quoted naval experts saying the data China is gathering would be invaluable for deploying its own submarines more effectively and hunting adversary submarines in contested waters.[1]
This is why the usual naval debate is now too narrow.
Most discussion still focuses on fleet size, submarine numbers, missile ranges, and shipbuilding tempo. Those all matter. But a navy does not operate in abstraction. It fights in a physical environment. Undersea warfare is not only about the platform. It is about who knows the water column, who understands sonar performance in specific areas, who can place seabed sensors in the right places, and who can move through underwater terrain with more confidence than the other side.[1][2]
China appears to have understood that.
And it appears to be acting on it at scale.
Cause
The cause is straightforward.
China wants to break out from the geographic and acoustic disadvantages of its near seas and prepare for high end undersea competition beyond them.
Reuters found that Chinese survey activity has concentrated on waters east of the Philippines, around Guam, around Hawaii, near Wake Atoll, in approaches to the Malacca Strait, and west and north of Alaska.[1] Those are not random locations. They sit along the routes through which American and allied submarines would move, the choke points through which Chinese shipping and submarines would pass, and the wider maritime approaches to the First Island Chain.
That chain is central to Beijing’s strategic problem. The Pentagon’s 2025 report says China’s military focus is currently the First Island Chain running from Japan to the Malay Peninsula, which Beijing sees as the strategic centre of gravity for its regional goals.[2] Reuters quoted former Australian naval officials saying China is deeply concerned about being boxed into that chain and that its seabed mapping indicates a desire to understand the maritime domain well enough to break out.[1]
This is not only about navigation.
It is about undersea control.
Rear Admiral Mike Brookes told the U.S. China Economic and Security Review Commission in March that China views undersea warfare as an enabler across the full spectrum of naval operations and is integrating air, surface, seabed, and undersea sensors into an anti submarine warfare system of systems.[3] He said China is building layered undersea surveillance networks that gather hydrographic data such as water temperature, salinity, and currents in order to optimise sonar performance and enable persistent surveillance of submarines transiting critical waterways.[3]
That is the deeper shift.
China is not merely trying to build submarines that can survive in the undersea domain.
It is trying to build an undersea domain that is more favourable to Chinese submarines and more dangerous for everyone else.
Reuters traced this effort through five years of movement by 42 research vessels active in the Pacific, Indian, or Arctic oceans.[1] It found at least eight of those vessels had conducted seabed mapping and another ten carried equipment used for mapping.[1] Around 2014, Chinese scientists proposed a “transparent ocean” concept to create a comprehensive view of water conditions and movement through specific areas, and Reuters reported that the project quickly received at least $85 million in support from the Shandong provincial government.[1]
This is not opportunistic collection.
It is a sustained state backed programme.
Constraint
This is where the piece gets harder.
Because mapping the seabed is strategically valuable, but it does not instantly create undersea dominance.
The first constraint is technical complexity. Undersea warfare remains one of the most difficult military environments in the world. Sound propagation shifts with underwater terrain, water temperature, salinity, and currents.[1][3] Building a useful picture is hard enough. Maintaining it across vast ocean spaces and turning it into real time operational advantage is harder still.
Reuters noted that some experts expressed caution about Chinese claims that its sensor networks provide real time data on water conditions and subsea movement, because underwater communication remains technically difficult.[1] Even so, they also noted that delayed data is still valuable because it helps model conditions, optimise sonar, and improve the chances of detecting submarine operations later.[1]
The second constraint is scale.
The ocean is still largely unmapped even now. NOAA says that as of June 2025 only 27.3 percent of the global seabed had been mapped with modern high resolution technology.[4] That figure matters because it shows how powerful a targeted mapping campaign can be. A state does not need to map every ocean floor. It only needs to map the routes, ridges, choke points, and patrol areas that shape conflict.
China appears to be doing exactly that.
Reuters reported mapping around the Ninety East Ridge in the Indian Ocean, around the Luzon Strait area between Taiwan and the Philippines, around Guam, and around routes linked to Australian and American submarine movement.[1] That is selective preparation, not global curiosity.
The third constraint is that knowledge alone still has to be converted into capability. Rear Admiral Brookes said China currently operates over 60 submarines and could reach roughly 70 by 2027 and up to 80 by 2035, with about half nuclear powered by then.[3] He also said the PLA Navy is investing in unmanned undersea vehicles, seabed systems, and undersea surveillance networks to erode U.S. undersea advantages.[3] In other words, the mapping effort matters because it is tied to a wider undersea build out.
Without that link, bathymetric data is interesting.
With that link, it becomes part of a warfighting system.
The fourth constraint is visibility and attribution. China’s survey vessels often operate as civilian or university platforms. Reuters said the Dong Fang Hong 3 is operated by Ocean University of China and publicly described some of its work as mud surveys and climate research.[1] That is exactly what makes the programme strategically awkward. It sits inside civil military fusion. It can be presented as science while still feeding the military requirement for subsea intelligence.
That complicates response.
A country can contest a warship more easily than it can contest an oceanographic expedition that has both civilian and military value.
The fifth constraint is that the United States and its allies are not starting from zero. Reuters noted that America recently overhauled its own efforts to map and monitor the ocean, although it typically does so with military vessels that can switch off public tracking systems.[1] The long standing American advantage has not vanished. But Reuters also quoted Naval War College specialist Ryan Martinson saying that for decades the U.S. Navy could assume an asymmetric advantage in knowledge of the ocean battlespace and that China’s effort threatens to erode that advantage.[1]
That is the pressure point.
This is not yet proof that China owns the undersea domain.
It is proof that the domain is becoming more contested than many still assume.
Consequence
If China keeps expanding this effort, the consequences are strategic.
The first consequence is a narrowing undersea information gap. Brookes testified that by 2040 China’s undersea forces may credibly challenge U.S. regional maritime dominance, and that advances in submarines, sensors, seabed systems, and unmanned vehicles will raise the cost of U.S. operations in the western Pacific.[3] That warning becomes more serious when paired with Reuters’ reporting on actual mapping and monitoring in the waters where a future campaign would be fought.[1]
The second consequence is greater risk to allied submarine movement. Reuters reported that China has deployed advanced sensors in parts of the strait between Taiwan and the Philippines through which U.S. submarines would move to reach the South China Sea.[1] It also found Chinese survey activity around Guam, Hawaii, and Christmas Island, locations tied directly or indirectly to American and allied undersea operations.[1] If that data set grows and is fused effectively, allied submarines could face a less permissive environment before a crisis even begins.
The third consequence is pressure on AUKUS thinking. AUKUS is often framed through platforms, hull numbers, and industrial schedules. But undersea competition is not only about who builds submarines. It is about who can operate them with confidence through contested acoustic terrain. Reuters noted that HMAS Stirling is due to host a rotational force of up to four U.S. Virginia class submarines from 2027.[1] If China is already mapping routes and surrounding waters linked to allied undersea movement, then seabed intelligence has become part of the AUKUS operating problem, not a separate scientific issue.[1]
The fourth consequence is the militarisation of civilian ocean science. Reuters traced vessels back to Chinese ministries, universities, and state affiliated research institutions, and linked the effort to Beijing’s civil military fusion approach.[1] That means the undersea competition is no longer confined to navies and defence ministries. It now includes universities, research institutes, mineral exploration, sensor arrays, and dual use data collection. A state that ignores that fusion model will understate the scale of the challenge.
The fifth consequence is that naval mass alone becomes an incomplete measure of power. China’s current force already includes over 60 submarines, with projections of roughly 70 by 2027 and up to 80 by 2035.[3] But the more important development may be that these platforms are being paired with underwater gliders, sensor arrays, unmanned systems, and a growing body of bathymetric and hydrographic data.[1][3] The side that combines platforms with environmental mastery will have an advantage over the side that counts platforms only.
The sixth consequence is that seabed competition spills into economics and infrastructure. Brookes’ testimony said China is pursuing seabed systems and deep ocean capabilities in ways that create vulnerabilities for undersea cables, sensor networks, and other critical infrastructure relied on by the United States and its allies.[3] His testimony also noted that China holds five of the International Seabed Authority’s 22 exploration permits through state linked firms and is using deep sea mining activities in ways that may support military intelligence collection.[3] That means the seabed is not simply a naval arena. It is becoming a strategic layer where military advantage, infrastructure vulnerability, and resource access overlap.
Conclusion
China is not only building a larger navy.
It is trying to build a more intelligible ocean.
That is a serious distinction.
A navy that knows the seabed, understands local sonar conditions, places sensors first, and maps the submarine approaches of its rivals before war begins is not merely preparing platforms. It is preparing the environment in which those platforms will fight.[1][3]
That is why this issue should not be dismissed as ocean science with military side benefits.
It is strategic preparation in the undersea domain.
And it is happening now.
The next naval arms race is not only in shipyards.
It is on the seabed, in the water column, and inside the sensor networks that make oceans more transparent for one side and more dangerous for the other.[1][3][4]
When the next undersea crisis comes, which navy will be operating in contested waters and which one will be operating in waters it has already spent years quietly preparing?
References
[1] Reuters, “China is mapping the ocean floor as it prepares for submarine warfare with the U.S.,” 24 March 2026.
[2] U.S. Department of Defense, 2025 Annual Report to Congress: Military and Security Developments Involving the People’s Republic of China, on China’s focus on the First Island Chain and long term world class military ambition.
[3] Rear Admiral Mike Brookes, testimony to the U.S. China Economic and Security Review Commission, 2 March 2026, on China’s undersea warfare strategy, submarine force, seabed systems, and projected force growth.
[4] NOAA Ocean Exploration, “How much of the ocean has been explored?” stating that 27.3 percent of the global seabed had been mapped with modern high resolution technology as of June 2025.
China Is Mapping the Ocean Floor for War was originally published in DataDrivenInvestor on Medium, where people are continuing the conversation by highlighting and responding to this story.
