During the war, the first hint was discovered. During World War II, sonar operators on U.S. Navy ships stared at their instruments, reading what appeared to be the ocean floor 300–500 meters below the surface. Everything was normal, but the seafloor continued to move. The men at the screens had no idea why it was deeper during the day and shallower at night. According to reports, some people claimed to have found submerged islands. They hadn’t. They were monitoring enormous, nearly unfathomable amounts of life.
The Deep Scattering Layer, or DSL, is the new term for that phenomenon. This area of the water column is so full of marine life that it reflects sonar signals back to the surface in a manner similar to that of the ocean floor itself. The primary offenders are fish with swim bladders, especially lanternfish, which are tiny, bioluminescent organisms that make up up to 65% of the world’s deep-sea fish biomass. When you aim sonar at them, the signal is not transmitted. It returns. And that appears to be solid ground to the instruments.
It’s worth taking a moment to reflect on that scale. The global biomass of lanternfish was previously estimated by scientists to be between 550 and 660 million tonnes, which is already a difficult number to remember. Then, in 2007, more accurate sonar surveys significantly increased that figure to between 5 and 10 billion tonnes. It’s not a rounding error. That is a completely different order of magnitude, and it implies that we were essentially speculating about one of the highest concentrations of vertebrate life on Earth for decades.
Additionally, the DSL does more than just sit there. It is made up of organisms that migrate upward every night, rising toward shallower, food-rich water to feed on plankton after sunset. They retreat into colder, darker depths at dawn, making it more difficult for predators to locate them. Although this behavioral loop has existed for millions of years, it wasn’t until humans began bouncing sound waves through the ocean that its side effect—a moving, breathing false seafloor—became an issue. Even moonlight affects the layer. It sits deeper on bright nights. Clouds shift upward when they roll over.
Research on the DSL has become much more sophisticated in the modern era. Kelly Benoit-Bird, a marine acoustician, and her colleagues sent an autonomous underwater vehicle—basically, a deep-diving robot equipped with sophisticated sonar—down through the layer in 2013 to examine its contents up close. They discovered something more organized than anyone had anticipated. The layer was not a disorganized soup of mixed marine life, but rather distinct groupings: fish and squid clustered apart, and individual animals were arranged in what appeared to be social order. Inside the layer, dolphins had been observed hunting. There was actual complexity.
Many things are still poorly understood. The DSL forms differently depending on the season, varies greatly between ocean basins, and exhibits behaviors that are beyond the scope of current acoustic technology. Despite significant advancements in military sonar systems since World War II, the layer continues to be a source of interference. This inherent characteristic makes it difficult to track submarines, navigate underwater, and map acoustics in ways that defense engineers are still trying to overcome rather than completely resolve.
Sitting with all of this, it seems odd that something so massive could have remained hidden for so long. Large portions of the world’s oceans are covered by the DSL. Every night, it shifts. According to some estimates, it has more living mass than practically anything else in the ocean. For many years, it was merely an incomprehensible line on a sonar screen that no one could fully comprehend. It turns out that the ocean is quite adept at keeping secrets, even ones composed of billions of fish.
