The remote expanse of the world’s oceans may seem like a tranquil space to conduct science. But sometimes, geopolitical tensions cross paths with scientific missions—and researchers get caught in the middle. On Thursday, Chinese sailors grabbed a submersible drone that was recording data for a US naval research ship operating in the volatile South China Sea.

The five-foot long torpedo-shaped instrument is called a Slocum glider. Except for the fact that it operates underwater, it resembles a yellow model airplane, steering itself through the water with wings, a tailfin, and modifiable internal buoyancy. As it does so, the glider records ocean temperature, salinity, and density—vital signs that can give oceanographers a picture of short-term weather conditions and longer-term climate patterns. The US Navy uses this data to help develop meteorological forecasts for ships crossing the world’s oceans. It owns 130 such gliders, about a third cruising the world’s oceans at any given time. But this week’s conflict isn’t the first time that a scientific instrument has been caught up in a maritime cat-and-mouse game between America and its naval adversaries.

The latest incident began when watchers aboard the USNS Bowditch—a naval research vessel—saw a Chinese Dalang III-Class salvage ship launch a smaller boat. Sailors on the small craft snatched the glider out of the water and took it back to the ship. The American commanders called their counterparts on a bridge-to-bridge radio. The Chinese acknowledged the call, but refused to answer, took the glider, and left the scene.

The US Navy wanted its drone back. “The UUV is a sovereign immune vessel of the United States,” Pentagon spokesman Peter Cook said in a statement released Friday. “We call upon China to return our UUV immediately, and to comply with all of its obligations under international law.” On Saturday, the Pentagon reported that China had pledged to return the drone.

The USNS Bowditch was surveying the South China Sea, a region that has seen conflicts between China and its neighbors for years. Researchers—and not just those employed by the Navy—spend time in this region because it is a crucial area for understanding how climate patterns like El Niño move heat from one side of the Pacific Ocean to the other. The region is also home to rare coral reefs, many of which are currently dying off from increasing ocean acidity caused by climate change.

But the region is also a geopolitical pressure cooker. Begin with the base: China and the US have been disputing political dominance in Southeast Asia for decades. Add the conflict between China and Taiwan. Season with an ongoing struggle for who controls the shipping lanes through the South China Sea. Sprinkle in the Spratly Island problem—a cluster of oil-rich, strategically-placed, mostly-submerged reefs being occupied by military forces from five different countries, and you've got yourself trouble stew.

America’s scientific instruments have been vulnerable in the open ocean for decades, and were sometimes sabotaged for security reasons, resale value or perhaps leverage, according to Jake Hamblin, professor of the history of science at Oregon State University and author of Oceanographers and The Cold War.

Several years ago, oceanographer Luca Centurioni from Scripps Institute in La Jolla, CA, was monitoring an array of five instruments moored to the ocean floor in the East China Sea. As he and his colleagues approached, two Chinese fishing boats suddenly appeared, racing them to the moorings. “They approached the instruments and tried to grab them and do various maneuvers to prevent us from completing the operation,” Centurioni says. “It went on for an hour and a half.”

Only when a Taiwanese support vessel blocked the fishing boats from the moorings did they leave. Centurioni believes they were trying to grab the instruments and perhaps sell them to the Chinese government. “It’s not clear what they can and cannot do when you have something in the ocean,” he says. “In theory, it’s anyone’s property.”

Another oceanographer named Russ Davis remembers mixing it up with the Soviets back in the 1980s. At the time, he was measuring surface currents aboard a research vessel anchored a few hundred miles north of Hawaii. For three days, a Soviet ship circled the oceanographers. “We called them up on the radio,” said Davis, a professor emeritus at Scripps. “They refused to respond. We made an open invitation for them to come over and join us.” The eerie standoff continued until a US Naval P-3 Orion surveillance plane flew overhead, dropping supplies and equipment from Hawaii.

And though this oceanographic pseudo-cold war has been going on for decades, the current situation is somewhat different. For one thing, the Chinese military isn't just haranguing at random. According to satellite images released this week, it has begun installing anti-aircraft weapons on several of those disputed semi-submerged coral reefs. And even though the Japanese aren't involved in the Spratly Island territorial dispute—it's between Vietnam, the Philippines, Taiwan, and Malaysia—Chinese patrol boats have also squared off against the Japanese coast guard.

In a lot of cases, the offending country accuses the other of spying. But the US contends that this instrument is not cover for any clandestine operations, and that it is only collecting naval oceanographic data. “At the very least it is a message that Chinese overtures cannot be ignored,” Hamblin said in an email. “These kinds of acts will likely continue as long as there are territorial disputes over the South China Sea.”