The concept of a network of hydrophones sitting thousands of meters below the surface of the ocean, silently recording every tremor and groan of the Pacific without anyone having to be there, is almost unsettling. Not a ship. Not a crew. Just sound, passing through chilly, dark water, and reaching a sensor that records everything without complaining. This is precisely what the new passive acoustic monitoring systems currently undergoing testing and deployment throughout the Pacific basin accomplish. The implications for tracking submarine volcanic activity are so profound that scientists who have dedicated their careers to the issue are using terms like “game-changing.”
Even though the technology wasn’t always in favor of humanity, the physics has always been. After entering what oceanographers refer to as the SOFAR channel, a low-velocity zone usually found between 500 and 1,000 meters below the surface, sound travels hundreds to thousands of kilometers with surprisingly little signal loss.
Sound travels about four times faster through seawater than through air. This implies that a hydrophone anchored off the coast of Oregon could detect a volcanic event close to Tonga under the correct circumstances. Since the early 1990s, when scientists first used military hydrophone arrays from the Cold War and captured the first-ever real-time recording of a deep-sea volcanic eruption in June 1993, NOAA’s Pacific Marine Environmental Laboratory has been aware of this. In some ways, that moment was almost accidental, but it proved what physics had always predicted.
Scale and autonomy are now different. Previously requiring ship deployments and months-long retrieval cycles, moored hydrophone systems have become more intelligent and networked. Over the course of more than 20 years, PMEL’s acoustics researchers have developed a variety of recording technologies, including mobile platforms, satellite-linked surface buoys, and autonomous stationary hydrophones, and deployed them throughout all of the major ocean basins. With the Pacific receiving special attention due to its seismic and volcanic restlessness along almost its whole rim, the result is something close to a continuous, planet-wide listening network.
Ocean gliders are now a significant component of this image. These buoyancy-driven autonomous platforms are ideal for passive acoustic work because they move silently through the water column with no engine rumble or propulsion noise. Less than 13% of time at sea was unsuitable for monitoring because of surface communications or maneuver noise, according to a study using data from 34 PAM glider missions. For any remote sensing platform operating in such a physically demanding environment, that is an exceptionally high data-yield figure. It implies that the glider approach may eventually supplement fixed moorings in monitoring submarine volcanic chains that are just too long, too far away, or too expensive to cover with anchored systems alone.
A sense of what’s possible can be found in the southwestern Pacific’s Tofua Arc. Over the course of 15 months starting in early 2009, research monitoring the acoustic responses of submarine volcanoes along that arc discovered stationary, persistent sound sources whose signatures were recorded. Researchers were able to map directional patterns in the noise field and differentiate volcanic sources from background oceanic noise thanks to the signals, which were low-frequency rumbles that were mostly below human hearing. It’s an intricate, laborious science, but the patience seems justified when the alternative is to discover an eruption only after it causes a tsunami.
Observers in this field are struck by how much the technology has quietly advanced while the public’s focus has remained on more dramatic ocean stories. Some of that focus was refocused by the 2022 Tonga volcanic eruption and its unusual tsunami, which viscerally illustrated how little is known about submarine volcanic behavior and how quickly monitoring gaps can have real-world repercussions. Acoustics researchers believe that this eruption was an unintentional justification for their work; rather than grant proposals, geology provided proof of concept.
It is genuinely unclear if full Pacific-wide simultaneous monitoring will become operational standard within the next ten years. There are actual costs, logistical issues, and data processing difficulties. However, the trajectory appears sufficiently clear that it is more difficult to maintain skepticism than it was even five years ago. There has always been conversation on the ocean floor. Perhaps we are just now developing ears capable of hearing everything at once.
