All the Fish We Did Not See: How a WWII Sonar Glitch Off San Diego Revealed the Ocean’s False Bottom
In 1942, Navy sonar off San Diego found a mysterious ‘false bottom’ hundreds of feet down. It turned out to be alive: the deep scattering layer, part of the largest animal migration on Earth.
Updated
In the summer of 1942, a Navy ship called the USS Jasper was off San Diego testing a new weapon against Japanese and German submarines: long-range active sonar. The idea was simple. Send a pulse of sound into the water and listen for the echo. If it bounces off something, like a submarine or the seafloor, you know it’s there.
The work was led by Carl F. Eyring, a physicist who specialized in sound, along with Ralph A. Christensen and Russell W. Raitt.
The USS Jasper in 1945—just a few years after scientists discovered the first evidence of the Deep Scattering Layer during a research cruise aboard the ship. (Photo: Naval History and Heritage Command)
Then something weird showed up. About 300 yards down, in water that was supposed to be much deeper, the sonar kept bouncing off something. It looked like the seafloor had suddenly risen up under the ship.
That made no sense. The charts said the bottom was much deeper, and the echoes were strong and jumpy, as if something big was in the way.
Carl F. Eyring (Brigham Young University)
But there was a war on. The scientists measured the mystery layer, figured out how to work around it so it wouldn’t hide submarines, and moved on. Figuring out what it actually was would have to wait.
In 1945, the oceanographer Martin Johnson dropped nets into the Pacific and solved it. The “bottom” was alive. It was a huge cloud of animals, most no bigger than your finger, and it moved up toward the surface every night and back down every morning. The sound was bouncing off their gas-filled swim bladders. There were so many of them that, to the sonar, they looked like solid ground.
Bristlemouth trawled from the ocean’s twilight zone (Erik Olsen)
It became known as the Deep Scattering Layer, the ocean’s “false bottom.” And it upended what scientists thought about the middle depths of the ocean, which were supposed to be pretty empty. Instead, they were packed with fish, squid and tiny drifting animals.
The layer lives in the mesopelagic zone, better known as the twilight zone, from about 650 to 3,300 feet down, where there’s only a faint blue glow of sunlight. And that daily up-and-down trip, called diel vertical migration, is the biggest animal migration on Earth. It happens every single day. Each evening, billions of animals (some scientists think quadrillions), including little fish like lanternfish, hatchetfish and bristlemouths, swim up to feed near the surface in the dark, then sink back down at dawn to hide from predators. (Bristlemouths, by the way, may be the most numerous vertebrates on the planet. You’ve probably never heard of them.)
Scattering layer seen on sonar (Erik Olsen)
It turns out the migration matters for the whole planet. These animals eat near the surface at night, then go back down and poop, and eventually die, in deep water. That carries carbon from the surface down into the deep ocean, where it can stay locked away for centuries. Scientists call it the biological carbon pump, and it’s a big part of how the ocean pulls carbon dioxide out of the atmosphere and helps keep the climate in check.
Even now, eight decades after the Jasper, the twilight zone is one of the least explored parts of the ocean. Some of the most ambitious work is happening at the Woods Hole Oceanographic Institution, where scientists are going after it with robots, submersibles and new acoustic tools.
Heidi Sosik uses automated cameras and imaging systems to study who lives in the twilight zone and how much carbon their daily commute moves into the deep.
Bristlemouth fish (Erik Olsen)
Andone Lavery does what Eyring’s team did in 1942, only with far better equipment. She uses sound to work out what’s actually in the scattering layer and how the animals behave, which matters a lot for estimating how much life is down there.
And Simon Thorrold uses chemical tracers to follow food through the twilight zone. His work shows how much big predators like tuna, swordfish and whales depend on the little fish coming up from below.
Scientists use a Triton submersible to explore the ocean’s twilight zone in the Bahamas. (Erik Olsen)
WHOI’s Ocean Twilight Zone project even built its own towed instrument platform, the Twilight Zone Explorer, or TZEX, to survey the layer. There’s some urgency. With that much fish down there, commercial fishing companies are starting to look at the twilight zone, and scientists like Sosik want to understand what we’d lose, both in the food web and in carbon storage, before anyone starts hauling it up.
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Not bad for a wartime glitch. A Navy crew off San Diego was looking for submarines and instead stumbled onto the largest migration on the planet, happening right under their ship every night.
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