In ocean research, there is a moment that keeps happening in different labs on different continents by scientists who all expected the same thing and received the same surprise. They wait for silence while lowering a microphone thousands of feet or almost seven miles into the depths. They never understand. When researchers from NOAA and Oregon State University lowered a titanium-encased hydrophone into the Challenger Deep of the Mariana Trench, past 36,000 feet of crushing pressure and complete darkness, what emerged was more akin to a traffic intersection than a void. There is an earthquake. Baleen whales’ low moan. The whine of a container ship’s propeller. The complete acoustic signature of a category 4 typhoon passing overhead, of all things.
The air pressure in a normal office is less than 15 pounds per square inch, whereas the pressure at Challenger Deep is over 16,000 pounds per square inch. Years of engineering work were needed to build a microphone that could withstand that environment without imploding, and even then, the device had to be lowered at a carefully regulated pace to avoid being crushed on the way down. After three weeks of recording, chief scientist Robert Dziak described the results as “almost constant noise from both natural and man-made sources.” It’s possible that no one involved in the project fully anticipated being taken aback. However, they were.
As the field of ocean acoustics grows, that surprise has become somewhat of a theme. The world’s oceans now have thousands of hydrophones installed, providing data to scientists who are just starting to understand what they are hearing. After 25 years of listening to coral reefs, Steve Simpson, a marine biology professor at the University of Bristol, claims that putting a microphone in the water completely transforms the experience. “When we put an underwater microphone into the reef it comes alive, giving us a whole new dimension,” he claims. The sharp crackle of snapping shrimp, the territorial chirps of clownfish protecting their anemones, the pops and clicks of sergeant major fish chasing intruders away from their nests—all of these sounds that the human ear misses while diving become a rich, multi-layered audio record when played back through a hydrophone.
The quantity of microphones is not the only thing that distinguishes the current state of ocean acoustics from previous decades. Once the recordings are obtained, researchers can use them for this purpose. What analysts were referring to as the primary bottleneck in the field—the utter impossibility of having human researchers sit with headphones and manually catalog months of audio—has been eliminated by artificial intelligence. In the time it would take a graduate student to review an afternoon’s worth of recordings, machine learning can now process ten. According to Jill Munger, a marine acoustics analyst at Conservation Metrics, it is now possible to replicate and compare at a scale that was previously unthinkable. The dataset gets richer the more instruments are placed in the water, and the richer the dataset, the more AI can discover in it.
A portion of its findings are still inexplicable. In Kāne’ohe Bay, on the windward side of O’ahu, Annie Innes-Gold, a PhD researcher at the Hawai’i Institute of Marine Biology, has been constructing artificial reefs out of cinder blocks and observing what moves in. A sound that researchers have dubbed the “cascading saw” can be heard on several of her recordings. It is a repeating, trumpeting call that appears at her locations, at Palmyra Atoll, and possibly in the Red Sea, though its source is unknown. It might be a fish. It might be an unidentified species. It might be a completely new way for marine life to produce sound. No one is aware. Since scientists estimate that about 90% of ocean species are still undiscovered, these mystery recordings can be viewed as glimpses of something that is just waiting to be discovered rather than gaps in the data.
Sitting with all of this gives us the impression that the ocean has been having a lengthy conversation that we have only recently begun to properly listen to. Simpson explains how hearing reefs die, with the ecosystem collapsing inward before the outward signs emerge and the sound becoming silent before the colors fade. In locations where marine protected areas allowed the ecosystem to recover, he has also heard them return. The fact that you can hear a reef regaining its health before you can see it is the kind of detail that completely changes one’s perception of what it means to monitor an ecosystem.
Naturally, the acoustic image isn’t totally natural. The underwater soundscape is filled with human noise that would not have been audible prior to the industrial era, such as shipping traffic, sonar, and industrial activity. According to scientists, there is very little similarity between the sounds of the ocean today and those of preindustrial times. These millions of hours of recordings are now positioned to address the question of whether marine animals are simply struggling beneath that noise or are adjusting to it.
