KAOHSIUNG, Taiwan—China has accelerated the mapping of waters near Taiwan, sending dozens of ships to gather data that could be critical for submarines and subsea drones to navigate and detect rivals in a fight over the self-ruled island.

The recent flurry of maritime research has the potential to advance China’s ability to seize Taiwan, which Beijing claims as its own and threatens to take by force if necessary.

Taiwan responded in August by adding two ships to its small research fleet as it works to map this potential maritime battleground while it invests in an arsenal of nautical drones to improve the island’s defenses.

“Only by gaining a more complete understanding of our own maritime environment can we safeguard national security,” Taiwan’s President Lai Ching-te said at the launch. But China is far ahead—in mapping, shipbuilding and assembling a drone arsenal.

In recent months, Chinese research ships have been particularly active to the east of Taiwan, surveying waters that the U.S. Navy would need to cross to intervene in a conflict over the island.

A Wall Street Journal analysis of ship-tracking data shows Chinese vessels sailing back and forth in a lawnmower pattern over swaths of ocean to Taiwan’s south and east, helping Beijing to complete its portrait of waters surrounding the island.

Taiwan’s coast guard said the work by Chinese research vessels aids “battlespace preparation.”Beijing’s Taiwan Affairs Office didn’t respond to questions about China’s research.

Chinese vessels routinely survey the sea floor and water conditions from the Pacific to the Indian Ocean, gathering data that can be used for science and sea mining but can also be put to military use.

China wants details of the bottom contour to help its submarines and underwater drones, which could be used in a blockade of Taiwan or to prevent U.S. intervention, said Bryan Clark, senior fellow and director of the Center for Defense Concepts and Technology at the Washington-based think tank Hudson Institute.

Undersea vehicles can use detailed maps of the sea floor to navigate autonomously without returning to the surface to link with satellite systems. That enables them to operate unseen for longer periods.

Chinese research around Taiwan increased in particular after Japan and the Philippines said in May that they would hold talks about overlapping maritime boundaries in waters to the east of Taiwan. The two countries have been increasing their defense cooperation in response to perceived threats from Beijing.

China, which claims the waters surrounding Taiwan as its own territory, called the talks illegal and launched a series of coast guard exercises to press its claim.

“China is capitalizing on the boundary talks as a pretext for what it likely wanted to do anyway: that is, to ratchet up the pressure on Taiwan,” said Allen Zhang, a research assistant at the Heritage Foundation think tank who has studied China’s research ships.

“Beijing has long sought legal cover for its gray-zone tactics—coercive activities short of lethal force—and research vessels fit that mold quite well,” Zhang said.

Though Taipei has an edge in mapping waters close to shore that are off limits to Chinese research vessels, Beijing has elevated oceanography to a national strategic priority, investing in ships and advanced underwater drones. China operates more than 120 research vessels, according to Taiwan’s coast guard.

“They are launching research ships as fast as dropping dumplings into a pot,” said Su Chih-chieh, an associate professor at National Taiwan University’s Institute of Oceanography, which operates one of Taiwan’s research ships. “Taiwan is having a really hard time catching up right now,” said Su.

Taiwan has only five ships actively carrying out its own maritime research: three managed by universities, one by the government-backed Taiwan Ocean Research Institute and one by Taiwan’s navy.

The two ships launched this summer will be run by yet another institute, under the government’s Ocean Affairs Council, which manages Taiwan’s coast guard.

“We want to take these fragmented pieces, gradually gather the data, add our own input, and build a marine database,” said Chen Chung-ling, president of the National Academy of Marine Research in Kaohsiung, which plans to launch its own large ship in 2029.

Chen describes Taiwanese research efforts as an expression of political will to safeguard Taiwan. “The more you understand the ocean, the more you can get from it,” she said.

But Taiwan’s research ships primarily serve academics with interests that aren’t necessarily aligned with defense needs, said Liao Te-yu, director general of the Taiwan Ocean Research Institute. “We actually understand very little about the waters surrounding us,” Liao said at the institute’s Kaohsiung headquarters.

Beijing’s research vessels are just one part of the Chinese maritime presence around the island. In mid-October, Taiwan identified six Chinese survey vessels, six Chinese coast-guard ships and four fishing boats that Taiwan believes are part of China’s maritime militia.

The ships were “clear examples of coordinated activities” between Chinese agencies, Lii Wen, deputy secretary-general of Taiwan’s National Security Council, tweeted at the time.

China’s research vessels are also used to document telecommunications cables, another potential target in a conflict, according to analysts.

“They’re trying to see what the undersea infrastructure looks like, because they probably want the ability to cut off Taiwan from outside communications if they can as part of a coercion campaign,” said Clark, a former U.S. Navy submariner. Chinese ships have severed Taiwan’s cables several times in recent years.

An October study by the Center for Strategic and International Studies found that China-linked research vessels on the high seas “spent a disproportionately large share of their observed time near critical infrastructure compared with research vessels from 47 other countries.”One Chinese ship, the Ke Xue, was active this summer near undersea cables northeast and southwest of Taiwan, said Jose M. Macias III, an associate fellow at CSIS’s Futures Lab. The same ship was seen in 2024 near cables in the Bering Sea and between Guam and Taiwan, the study found.