A network of microphones has been listening nonstop for decades, thousands of feet below the surface of the North Atlantic. They were never meant to look into the weather. It was planned that they would catch submarines.
During the Cold War, the U.S. Navy had a real fear that Soviet submarines could sneak across ocean basins without being seen and attack without warning. This led to the Sound Surveillance System (SOSUS). Starting in the early 1950s, the Office of Naval Research quietly paid for underwater microphone arrays to be put in place along the ocean floor. These arrays were then connected by cable to listening stations on land in both the Atlantic and Pacific oceans. Engineering in and of itself was very advanced for the time. The secrecy around it was even stronger.
People in the Navy were taught for decades how to block out what they called “biologicals”—the low, grinding moans and pulses that weren’t coming from any submarine. It turned out that what they were hearing were whales. Blue whales. Fin whales. Animals singing across whole ocean basins at frequencies that are so low that people can barely hear them. The Navy knew. They didn’t care. It was better to block out those sounds than to record them because they were just noise.
After the fall of the Soviet Union, that changed. Under a dual-use plan that Congress pushed through in 1991, the Navy started letting civilian researchers look at SOSUS data. As soon as scientists listened to those recordings, it became clear that the system could do a lot more than what it was designed to do. Oceanographers could now use hydrophone arrays set up off the coast of Oregon to find underwater volcanic eruptions off the coast of Japan. By studying the sound of a whale’s call, marine biologists could follow it across an ocean to different areas every hour. NOAA quickly saw the value and started using the arrays to follow the migrations of blue whales in the northeast Pacific.
It was almost natural for the climate application to come next. Sound moves through water in different ways depending on how warm or cold it is. When water is warmer, it changes the speed at which sound waves travel, and these differences can be measured over long distances. Scientists found that they could get a good idea of the temperature of large parts of the ocean by timing how long it takes for a sound to travel from a known source to a set of hydrophones. Acoustic thermometry is the name of the method used, and it’s the kind of tool that turns a Cold War artifact into something useful for figuring out how fast the deep ocean is warming up.
Rows and rows of old dot-matrix printers that used to print out information about submarine movements are now part of datasets that track one of the most important problems of this century. Cornell bioacoustician Chris Clark was the first person to walk into an SOSUS facility in 1992 and see a blue whale signature on a printout. He said it was a physically chilling experience. He walked into a room that was meant to help win a war and knew right away that it could be used for something else.
It’s still not clear how much of SOSUS is still working in its original form or how deeply the remaining arrays are connected to ongoing climate research projects. The Navy is keeping the technical details of these things secret. It’s clear that the system has outlived its original enemy by many decades and found a new use for itself that its creators probably never thought of. Every design choice was based on the threat of Soviet submarines. The science of climate change wasn’t even thought about.
Now, through those same cables and ceramic sensors, the ocean is telling us things that are harder to refuse. The water is getting warmer. Things are getting different. It turns out that a network that was made for one type of listening works really well for another.
