China is mapping the ocean floor as it prepares for submarine warfare with the U.S.  

China is extensively mapping ocean floors to enhance submarine warfare capabilities against the U.S. and allies across key oceans.

China is conducting a vast undersea mapping and monitoring operation across the Pacific, Indian and Arctic oceans, building detailed knowledge of marine conditions that naval experts say would be crucial for waging submarine warfare against the United States and its allies.

In one example, the Dong Fang Hong 3, a research vessel operated by Ocean University of China, spent 2024 and 2025 sailing back and forth in the seas near Taiwan and the ​U.S. stronghold of Guam, and around strategic stretches of the Indian Ocean, ship-tracking data reviewed by Reuters shows. In October 2024, it checked on a set of powerful Chinese ocean sensors capable of identifying undersea objects near Japan, according to Ocean University, and visited the same area again last May. And in March 2025, it criss-crossed the waters between Sri Lanka and Indonesia, covering approaches to ‌the Malacca Strait, a critical chokepoint for maritime commerce.

According to the university, the ship was carrying out mud surveys and climate research. But a scientific paper co-written by Ocean University academics shows it has also conducted extensive deep-sea mapping. Naval-warfare experts and U.S. ​Navy officials say the type of deep-sea data being collected by the Dong Fang Hong 3 – via mapping and placement of sensors in the ocean – is giving China a picture of the subsea conditions it would need to deploy its submarines more effectively and hunt down those of its adversaries.

The ⁠Dong Fang Hong 3 isn’t operating alone. It is part of a broader ocean mapping and monitoring operation involving dozens of research vessels and hundreds of sensors. In tracing this effort, Reuters examined Chinese government and university records, including journal articles and scientific studies, and analysed more than five years of movement by 42 research vessels active in the Pacific, Indian or Arctic oceans using a ship-tracking platform built by New Zealand company Starboard Maritime Intelligence.

While the research has civilian purposes – some of the surveying covers fishing grounds or areas where China has mineral prospecting contracts – it also serves a military one, according to nine naval-warfare experts who reviewed Reuters’ findings.

To gather information about underwater terrain, research vessels map the sea floor while traveling back and forth in tight lines. The ‌tracking data shows that type of movement by the vessels Reuters tracked across large sections of the Pacific, Indian and Arctic oceans.