Cold War submarine sensors revealed whale songs travelling across entire oceans

For years, some of the world's most sensitive underwater listening devices were tuned for one purpose: detecting Soviet submarines. Instead of the whirring of diesel engines and the pinging of sonar, they recorded an unexpected record of whale calls moving through the ocean.Back during the Cold War, the US Navy built an expansive ocean surveillance network called SOSUS, or Sound Surveillance System. Strategically placed across the Atlantic and Pacific, this network was designed to detect the subtle hums of Soviet submarines, giving naval forces early warning of submarine activity.What the Navy personnel monitoring the system didn't know, at first, was that they were inadvertently capturing something far older than any submarine. Haunting, low-frequency sounds pulsed and moaned across their recordings, some travelling across extraordinary distances.These sounds weren't from any machine. They were the songs of whales.A trans-oceanic listening networkThe SOSUS system emerged in the 1950s during an escalating period of tension between the US and the Soviet Union. The U.S. Naval Institute explains that the network consisted of rows of hydrophones, essentially, underwater microphones, bolted to the seafloor. These were meant to track the movements of submarines over enormous stretches of the ocean.The network became a powerful long-range listening tool.Cornell University's Bioacoustics Research Program reports that Navy operators frequently noted peculiar low-frequency sounds that didn't correspond to any known ship or submarine. These recordings were eventually labelled "biologicals," indicating they originated from marine life. The Cornell Chronicle reported how Clark and his colleagues realised the Navy's listening infrastructure was capable of detecting and following the calls of singing whales across entire oceans. Scientists gained a powerful new way to study whale communication over vast distances."We now have evidence that they are communicating with each other over thousands of miles of ocean," Clark told Cornell University in 2005.This discovery challenged previous understandings of how whales navigate and interact in their marine environment.Songs too deep for human earsWell, the sound detected was so low in frequency that humans could not easily hear them without processing. Subsequent analysis showed that many of these sounds were produced by blue whales and fin whales, two of the largest animals ever known to have lived.According to NOAA Fisheries, these baleen whales produce extremely low-frequency vocalisations that often lie below the threshold of human hearing. For the unversed, this ability to emit low-frequency sound is crucial for long-distance communication because low frequencies travel much more efficiently through water. A scientific discovery hidden in military archivesThe story of SOSUS remains a remarkable instance of accidental scientific discovery. A surveillance system created to track political adversaries inadvertently captured an unprecedented acoustic record of marine life.What started as a mission to detect submarines revealed the oceans to be teeming with long-distance conversations happening far below the reach of human ears.The SOSUS recordings helped transform scientists’ understanding of whale songs, showing they are not isolated sounds but parts of an acoustic world in which some whale calls can travel over enormous distances.