In the summer of 1942, aboard the USS Jasper, a team of scientists embarked on a mission off the coast of San Diego, California, a hub for U.S. Navy operations and other military activities vital for the Pacific Theater of World War II. Their goal was to test a new technology called “long-range active sonar,” developed to detect enemy submarines—specifically Japanese submarines and German U-boats—during World War II. Long-range active sonar is a technology that sends sound waves through the ocean to map and visualize the seafloor across great distances, revealing details of underwater topography and structures that would otherwise remain hidden beneath the waves.

The expedition was led by Carl F. Eyring, an accomplished acoustic physicist known for his pioneering work in sonar technology. Eyring, along with his colleagues Ralph A. Christensen and Russell W. Raitt, played crucial roles in the mission. Their combined expertise in acoustics, naval operations, and marine science made them the perfect team to explore the deep ocean with sound.

As they deployed sonar pulses into the depths, they encountered an unexpected anomaly: a persistent, dense layer approximately 300 yards (about 274 meters) below the surface that scattered their acoustic signals. It was almost as if the ocean floor had risen, looming closer with a strange, unyielding presence that defied all explanations.

This new reading was a complete anomaly, contradicting everything they knew about the seafloor’s topology. It was as though a solid mass had somehow materialized in the depths—a mass dense enough to obscure their sonar and make the familiar landscape unrecognizable. At the same time, their signal strength readings spiked erratically, suggesting significant interference in the water.

The discovery of this peculiar layer presented an intriguing puzzle to the scientists aboard the Jasper. Yet, with a war raging, they couldn’t afford to lose focus. Instead, they concentrated on measuring its dimensions and mitigating the acoustic interference it created. Determining its true nature would have to wait for another time.

It wasn’t until almost three years later, in 1945, that oceanographer Martin Johnson deployed nets into the Pacific and uncovered the truth: the layer was actually a massive cloud of marine animals, most no larger than a human finger, migrating daily from the deep ocean to the surface and back. This dense biological layer, packed with animals capable of reflecting sonar, had created the illusion of a solid mass, effectively “masking” the true depth of the ocean floor by reflecting sonar waves off the swim bladders of the fish and other marine organisms. 

This phenomenon, later termed the Deep Scattering Layer (DSL), created a “false bottom” in sonar readings, revealing an unexpectedly dense concentration of biological life in a mid-ocean zone once thought to be relatively sparse. The discovery of the DSL challenged assumptions about life distribution in the ocean, showing that vast numbers of organisms—such as fish, squid, and zooplankton—populate these depths, rising and descending with daily cycles to avoid predators and optimize feeding.

The DSL is situated within the ocean’s mesopelagic zone, commonly referred to as the twilight zone, which extends from about 200 to 1,000 meters below the surface. This region is characterized by minimal sunlight penetration and hosts a diverse array of marine life. Indeed, this huge swath of biomass is exactly what the sonar was picking up. This remarkable behavior observed in this zone is the diurnal vertical migration—the largest daily movement of biomass on Earth, the world’s largest animal migration. Each evening, billions of organisms (some scientists actually believe they number into the quadrillions) including small fish like lanternfish, hatchetfish and bristlemouths, ascend toward the surface to feed under the cover of darkness, retreating to the depths at dawn to evade predators. (Bristlemouths, by the way, are said to be the most numerous vertebrate on the planet.)

The discovery of the DSL provided significant insights into marine biology and oceanography. The layer’s composition—primarily swarms of marine animals with gas-filled swim bladders—explained the sonar reflections that mimicked the seafloor. This understanding highlighted the abundance and biodiversity of life in the twilight zone and underscored the importance of these organisms in oceanic ecosystems.

The discovery also led over time to an understanding of the role this layer plays in the carbon cycle, the very phenomenon that helps regulate Earth’s climate. The daily migration of marine animals in this layer is not just a remarkable biological spectacle; it is also a key mechanism for transporting carbon from the ocean’s surface to its depths. As these organisms ascend at night to feed and then return to deeper waters during the day, they excrete waste and many of them die, effectively moving carbon downwards, often sequestering it in the deep ocean floor where it can remain for centuries. This process, known as the biological carbon pump, plays a vital role in mitigating the effects of carbon dioxide in the atmosphere, thus contributing to climate stability. Without the existence of the Deep Scattering Layer and its role in the carbon cycle, the Earth’s carbon balance would be significantly different, highlighting just how interconnected marine ecosystems are with global climate regulation.

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In the decades following its discovery, the DSL has remained a subject of scientific inquiry. Advancements in sonar technology and deep-sea exploration have revealed the layer’s dynamic nature and its role in global carbon cycling.

Current research into the twilight zone, particularly by scientists at the Woods Hole Oceanographic Institution (WHOI), is uncovering fascinating insights into this enigmatic region of the ocean. The twilight zone remains one of the least explored parts of the ocean, despite being home to an abundance of life and playing a crucial role in global biogeochemical cycles. Woods Hole has been at the forefront of investigating this layer, employing advanced technology like remotely operated vehicles (ROVs), autonomous underwater vehicles (AUVs), submersibles, and cutting-edge acoustic techniques to understand its complex dynamics and ecosystem.

One of the leading researchers at WHOI, Dr. Heidi Sosik, has been focusing on the role that the twilight zone plays in the carbon cycle. Sosik’s work involves the use of automated imaging technologies to analyze the behavior and diversity of the organisms inhabiting this region. By documenting their daily migrations and interactions, Sosik’s team has been able to quantify the extent to which these animals contribute to carbon transport. This research is essential for understanding how much carbon is effectively being sequestered from the atmosphere through these daily migrations.

Another prominent scientist at WHOI, Dr. Andone Lavery, is working to map the twilight zone’s acoustics in unprecedented detail. Lavery’s expertise in underwater sound technology has helped reveal not only the composition of the Deep Scattering Layer but also the behaviors of its inhabitants. Lavery’s recent findings indicate that the twilight zone’s acoustic properties are far more dynamic than previously thought, and these properties can significantly affect how marine animals detect predators and prey, as well as how researchers measure biomass in this layer.

Dr. Simon Thorrold, also from WHOI, has been studying the food web dynamics within the twilight zone. Thorrold’s research has uncovered surprising insights into predator-prey relationships among mesopelagic species. Using chemical tracers, his team has been able to track the movement of nutrients through the food web, revealing that many animals from the twilight zone are integral to surface ecosystems as well, either through vertical migration or being preyed upon by larger species such as tuna, swordfish, and marine mammals.

In addition, WHOI has been collaborating with international partners on the “Twilight Zone Exploration” (TZX) project, which aims to better understand how human activities, such as fishing and climate change, are impacting this critical part of the ocean. The mesopelagic zone is increasingly targeted by commercial fishing due to the sheer biomass it holds. Dr. Sosik and her colleagues are actively studying the potential consequences of harvesting these species, considering their importance in carbon sequestration and as a key link in marine food webs.

Together, these efforts are gradually revealing the twilight zone’s secrets, emphasizing its importance not only in regulating climate but also in maintaining the health of marine ecosystems. As the pressures of climate change and human exploitation continue to grow, understanding this mysterious part of the ocean has never been more critical.

The USS Jasper‘s encounter with the false bottom off California’s coast stands as a pivotal moment in oceanographic history. It not only unveiled the hidden complexities of the ocean’s twilight zone but also bridged the gap between military technology and marine science, leading to a deeper appreciation of the intricate and interconnected nature of Earth’s marine environments.