The Underwater Arms Race: China Is Mapping the Seabed to Win the Next Submarine War

What Reuters Found

Reuters analyzed more than five years of movement data for 42 Chinese research vessels active in the Pacific, Indian, and Arctic oceans, using ship-tracking data from Starboard Maritime Intelligence, a New Zealand firm. The investigation also examined Chinese government and university records, journal articles, and scientific studies.

The core finding: the vessels were making the characteristic back-and-forth "lawnmower" movements across the sea floor that indicate systematic seabed mapping, concentrated in some of the most strategically significant stretches of ocean on Earth — near Taiwan, Guam, Hawaii, Wake Atoll, the Malacca Strait, and approaches to US military facilities in the Pacific.

At least eight of the vessels Reuters tracked have conducted seabed mapping operations. Another 10 have carried equipment used for mapping, according to Reuters' review of Chinese state media articles, vessel descriptions from Chinese universities, and government press releases.

The operation involves not just ships but hundreds of sensors placed in the ocean floor, Reuters reported. These sensors can identify subsurface objects — including submarines.

The Ship at the Center: Dong Fang Hong 3

Reuters focused in detail on one vessel as a case study: the Dong Fang Hong 3 (Chinese: "The East is Red"), operated by Ocean University of China. Ship-tracking data reviewed by Reuters showed the vessel spent 2024 and 2025 systematically sailing back and forth in the seas near Taiwan and the US military stronghold of Guam, and across strategic stretches of the Indian Ocean.

In October 2024, the Dong Fang Hong 3 checked on a set of powerful Chinese ocean sensors capable of identifying undersea objects near Japan, per Ocean University records. It returned to the same area again the following May. In March 2025, it cross-crossed the waters between Sri Lanka and Indonesia — covering approaches to the Malacca Strait, a critical chokepoint for maritime commerce.

Ocean University described the ship's activities as mud surveys and climate research. But a scientific paper co-written by Ocean University academics, reviewed by Reuters, showed it has also conducted extensive deep-sea mapping. Naval warfare experts and US Navy officials told Reuters the type of data being collected — via mapping and sensor placement — gives China a picture of subsea conditions it would need to deploy its submarines more effectively and hunt down adversaries' submarines.

Why Ocean Floor Data Matters for Submarine Warfare

Submarine warfare is fought in an environment that is inherently opaque. Unlike air or surface combat, submarines operate in water that bends, absorbs, and deflects sound in complex ways that depend on temperature, salinity, pressure, and the shape of the ocean floor. A submarine operating near an underwater canyon or ridge behaves acoustically very differently from one in open deep water. Knowing the exact topography of the seabed — and the three-dimensional sound-speed profile of the water column above it — allows a submarine commander to position their vessel in acoustic "shadow zones" that make detection difficult, or to exploit features that amplify the signatures of enemy submarines.

This knowledge is not static. It requires years of systematic survey data to build a reliable operational picture. The US Navy has spent decades accumulating this kind of environmental intelligence about the Pacific and Atlantic oceans. China is now building an equivalent dataset — and doing it in waters where the US has had longstanding dominance.

Peter Scott, a former chief of Australia's submarine force, told Reuters: "Any military submariner worth his salt will put a great deal of effort into understanding the environment he's operating in." He said the vessels' survey data "would be potentially invaluable in preparation of the battlespace" for Chinese submarines.

Jennifer Parker, an adjunct professor of defense and security at the University of Western Australia and former Australian anti-submarine warfare officer, was more direct in her assessment. She told Reuters: "The scale of what they're doing is about more than just resources. If you look at the sheer extent of it, it's very clear that they intend to have an expeditionary blue-water naval capability that also is built around submarine operations."

Civil-Military Fusion: China's Stated Policy

China's government has not denied that its research vessels map the seabed. It maintains the activities are for civilian scientific purposes. In the case of the Dong Fang Hong 3, Ocean University confirmed the ship carried out mud surveys and climate research, while also — per Reuters' review of published academic papers — conducting deep-sea mapping.

The dual use of civilian scientific data for military purposes is not an accidental byproduct in China's case — it is official policy. The Chinese government under President Xi Jinping formally promotes what it calls "civil-military fusion" (军民融合), the deliberate integration of civilian research and commercial activities with military technology development. Academic and government researchers are explicitly encouraged to work in ways that serve both scientific and national security objectives.

Parker and other experts told Reuters that even where data is gathered purely for scientific purposes, this integration means it is available to military planners in ways that it would not be in most Western countries.

China's Ministry of Foreign Affairs did not immediately respond to Reuters' request for comment on the investigation's findings, per the Reuters report.

Where the Fleet Stands

China's submarine force currently consists of approximately 66 submarines, according to Wikipedia's entry on the People's Liberation Army Navy Submarine Force. This includes both nuclear-powered and conventional submarines. By comparison, the US Navy operates approximately 71 submarines, according to The National Interest.

China's nuclear-powered submarine fleet has recently overtaken Russia's in size, making it the world's second largest after the United States, per Military Watch Magazine and Interesting Engineering in January 2026. China's shipbuilding capacity allows it to launch multiple nuclear hulls in parallel, and analysts project it will continue expanding its submarine force through the 2030s, per Interesting Engineering.

The US, by contrast, has faced production challenges. The Virginia-class submarine program has struggled with workforce and supply chain issues, according to Interesting Engineering, even as new boats were commissioned in 2025 and 2026.

The Strategic Context: Taiwan, Guam, Malacca

The geographic focus of the Chinese mapping operations — Taiwan, Guam, the Malacca Strait, approaches to Wake Atoll and Hawaii — maps almost perfectly onto the scenarios US military planners describe as the most likely theaters of a future US-China conflict. Any attempt by China to establish naval dominance around Taiwan in a conflict scenario would require controlling submarine access and transit through exactly the waters being mapped. Any attempt to prevent US reinforcement of Taiwan would require threatening the submarine and surface routes from Guam and Hawaii.

The Malacca Strait is separate but equally significant: it is the primary route for oil and trade between the Indian Ocean and the South China Sea. Control of or denial of passage through Malacca has been a long-standing element of Chinese strategic planning, and detailed knowledge of the approaches — which the Dong Fang Hong 3 was systematically surveying in March 2025 — would support both offensive and defensive submarine operations there.

What Is Not Confirmed

Reuters' investigation established the pattern and scale of China's ocean survey activities and obtained expert assessments of their military significance. What cannot be confirmed from open sources is the classified extent of US awareness of these activities, what US countermeasures if any have been taken, or whether any of the sensor networks identified by Reuters are already transmitting data to Chinese military planners in real time. China's government has not confirmed that the data collected by civilian research vessels is shared with military authorities, though civil-military fusion policy suggests that would be the intended outcome.