China is mapping the seabed to unlock a new edge in warfare

China’s fleet has triggered protests from Japan, India and others, but that hasn’t stopped Beijing from launching long missions to map the sea floor. The data they obtain has numerous applications, from science to the emerging industry of deep-sea mining, but it is the Chinese government’s commandeering of civilian research for military use that threatens American allies.

“If that data may have some value to the Chinese military and they want access to it, they’re going to get it," said Matthew Funaiole, senior fellow in the China Power Project at the Center for Strategic and International Studies. “There’s no firewall in place."

India’s navy drove a Chinese research ship away in 2019, but more have returned. In recent weeks, two of China’s most advanced research vessels, the Xiang Yang Hong 01 and the newer Dong Fang Hong 3 have conducted lawnmower-style sweeps across vast stretches of the eastern Indian Ocean.

Chinese civilian vessels are pursuing expeditions around the world—there are as many as a dozen or so active at any given time. The data they gather, including on currents, temperature and salinity, can have an array of applications. Detailed scanning of the seabed, for instance, can provide information about the visibility of naval mines and the accuracy of sonar.

The surveys can also help determine what minerals could be extracted. Beijing is already dominating the race to obtain the minerals critical for defense, electric vehicles and other technologies, and it is keeping ahead of Western commercial efforts to catch up.

In a study of 64 research and survey vessels operated by Chinese agencies, law enforcement, universities and state-owned enterprises, CSIS found more than 80% had links or showed behavior—such as visiting military ports—that suggested they were involved in “advancing Beijing’s geopolitical agenda."

Recent missions have spurred complaints by at least half a dozen governments. China says its marine research activities fully comply with the United Nations Convention on the Law of the Sea, the international treaty that established the legal regime for the world’s oceans.

New demand for hidden data

Understanding the sea floor and how sound travels through water at different depths is critical to whether submarines can be found or stay hidden.

“Sound will travel in different ways in different parts of the ocean, so understanding the contours of the ocean itself is important to understanding the acoustics," said Peter Dutton, a senior research scholar at the Paul Tsai China Center at Yale University.

The U.S. Navy also carries out maritime surveys for military purposes, although analysts say those efforts are more clearly separated from scientific research. Under the U.N. convention, coastal states have jurisdiction over research in their exclusive economic zones, which extend 200 miles from the coast. The U.S., which recognizes but hasn’t ratified the convention, argues that military surveys are exempt and U.S. Navy oceanographic ships regularly conduct studies near China.

China appears to be moving closer to the American stance, “as they’re doing more oceanographic research without the permission of coastal states," Dutton said.

The Indian Ocean is again a flashpoint, after a Sri Lankan moratorium on visits by foreign research ships, prompted by India’s complaints, expired at the end of 2024. In January, the Xiang Yang Hong 01 left its home port of Qingdao, sailing south on what is expected to be a 10-week, 11,000-nautical-mile research voyage collecting “diversified on-site observation data and high-quality samples," according to a Chinese state media report.

The ship sailed on Feb. 6 into the exclusive economic zone that surrounds India’s Andaman and Nicobar Islands, on its way to waters southeast of Sri Lanka, according to data from MarineTraffic, a ship-tracking and maritime-analytics service.

When the ship, which is owned by China’s Ministry of Natural Resources, was launched in 2016, a newspaper run by the State Oceanic Administration said its missions would include “comprehensive observation in the field of military oceanography," according to research by Dutton and Ryan Martinson, a scholar at the Naval War College in Rhode Island. The ministry didn’t respond to a request for comment.

In 2020, the ship carried out weeks of surveys near Christmas Island, an Australian territory in the Indian Ocean northwest of Australia. An Australian military official said at the time that the ship was mapping an area frequently used by Australian submarines heading to and from the South China Sea, the Australian Broadcasting Corp. reported. The Australian military said it monitored the ship, which it said was operating legally in international waters.

The ship entered the Bay of Bengal last year just before India launched tests in the area of a new ballistic missile. Three other Chinese research ships—including one operated by China’s military for tracking launches and satellites—were operating in the Indian Ocean at the time.

Such vessels have raised concerns in India over their long stays in the region and their use of equipment, including unmanned underwater vehicles, to collect data with potential military applications, said Anushka Saxena, a research analyst with the Takshashila Institution, an Indian think tank.

“How the hydrological data is collected, combined with the blurred lines of civilian and military in China, comes together to concern India, which was why India was so vocal to support the Sri Lanka moratorium," she said.

Exploring for minerals

Alarms went off again last month when China and the Cook Islands said they had entered a comprehensive partnership that includes an agreement to cooperate on oceanographic research.

The Cook Islands, a country of 15,000 people whose exclusive economic zone covers over 700,000 square miles of the South Pacific, is one of the few nations to have issued permits for deep-sea mineral exploration—a first step toward extracting minerals from the seabed.

Rules on governing the nascent industry, which has security and commercial applications, are being worked on by the U.N.-affiliated International Seabed Authority. Some nations, including China, have already begun surveys with the authority’s permission.

Deep-sea mining focuses on small rocks on the ocean floor that contain vital metals such as cobalt, copper, manganese and nickel. The Cook Islands, a former British protectorate that relies on New Zealand for its defense, is believed to have an extensive supply of nodules rich in cobalt in its exclusive economic zone.

New Zealand, which has opposed deep-sea mining, complained last month that the islands had secured the partnership with China behind its back.

Less than two weeks later, New Zealand again voiced concerns—this time over Chinese naval drills in neighboring waters. The country’s defense minister called the drills a wake-up call.

Detailed mapping of the sea floor, even if it is for prospective deep-sea mining, could increase New Zealand’s vulnerability, analysts say.

“The idea of massive Chinese ships coming into Pacific waters and digging up the sea bed, there is a security component," said Graeme Smith, a senior fellow in the Pacific Affairs department at Australian National University. “To do this mining, you need to have a really good map of what’s under the water."

Write to Austin Ramzy at austin.ramzy@wsj.com