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You can go to linode.com/scishow to learn more and get a $100 60-day credit on a new Linode account. [♪ INTRO] In the 1940s, ships were equipped with a fairly new technology: sonar. And that sonar detected a bizarre phenomenon: The sea floor appeared to be just 300 meters below the ship in areas that had been documented as being much deeper.

To make things even weirder, the depth changed throughout the night. Researchers at the time gave this creepy phenomenon a name that sounds more like a regrettable Scooby-Doo episode than a marine biology term: The Phantom Bottom. Today, scientists call it the deep scattering layer, or DSL for short.

The DSL ranges from 300 to 500 meters deep depending on the time of day, and it’s made of a layer of marine life so dense and massive that it mimics the bottom of the ocean on sonar. DSLs are found around the world, but they don’t just exist to confuse sailors and scientists. They play a vital role in the health of the ocean and the climate on Earth.

You’ll find lots of creatures in this layer spanning several kilometers in width and hundreds of meters in depth. So it’s no wonder that many ocean animals depend on it for their next meal. The creatures that make up the DSL include the tiniest microbes, crustaceans, gelatinous creatures like jellyfish, and countless fish species.

Lanternfish may be the most common larger creature found in these layers, maybe even the most widely distributed fish species on Earth. These small creatures make their own light, a handy tool when you live in the depths of the ocean where illumination is scarce. And it’s the swim bladder of all the fish living here, including the lanternfish, that strongly reflects sound waves from sonar, creating that eerie Phantom Bottom effect.

Now, a massive group of animals all hanging around in one spot may seem like an easy target for predators. But these critters have a few tricks up their sleeves to keep themselves safe. Researchers have found that DSLs aren’t uniformly mixed, with a shrimp here and a fish there and a squid over there.

Rather, species will group together, forming a layer within the layer. Scientists have observed a large patch of krill hanging out, and then another large patch of lanternfish swimming together, with zero mixing between the two. Researchers believe this might make it harder for predators to single out an individual creature.

A nifty way of hiding in plain sight. Not only will you find massive groups of single species hanging out together, they also move in unison up into shallow water at night and down into deeper water during the day. This is called diel vertical migration.

It’s another way to stay safe from predators. The deeper you go in the ocean, the less light there is, making it easier for animals to hide in the depths. But they still have to eat, and due to the lack of sunlight in the deeper ocean waters, there isn’t much in the way of food.

Some of these animals eat phytoplankton, microscopic algae that stay in the shallow layer of the ocean to photosynthesize. To get that, the DSL creatures need to swim up at night to grab a meal. Their predators follow along, and the predators of those animals follow, and so on until the entire DSL moves, like a perfectly orchestrated symphony timed with the sun.

It’s like an entire layer of the ocean is doing the wave. That’s some pretty impressive choreography. DSLs aren’t just important as ecosystems, though.

They play a critical role in the marine carbon cycle known as the biological pump, which keeps our planet from overheating. The biological pump moves some of the carbon produced in shallow waters to the deep sea in the form of marine snow. Made up of tiny carbon particles, marine snow is mostly animal poop and dead plankton.

Some of that marine snow gets munched on by the animals living in the DSL. This ends up moving a lot of carbon out of the shallow water and into the food web. For us, that’s better than it making its way back into the atmosphere as carbon dioxide, contributing to our planet’s rising temperatures.

Currently, DSLs on Earth are understudied, and there’s a lot left to learn about their contributions to Earth’s climate cycle. However, research suggests that these ecosystems are transforming due to climate change. As Earth warms, so does the ocean.

And as the ocean absorbs more heat, it’s becoming increasingly stratified, with the layers of the water unable to mix together because the temperature difference changes the density of the water. This prevents nutrient-rich bottom waters from moving to the surface and feeding phytoplankton. On top of that, low to zero oxygen zones are becoming more common as water temperatures rise.

These are spots in the ocean where there’s less oxygen available to the animals living there, because warmer water holds less oxygen than cold water. This makes certain locations in the ocean inhospitable to life. By the year 2100, the ocean is expected to look a lot different than it does today, and this will affect DSLs around the world.

In some places, there will be very little phytoplankton left and the DSLs in those regions will decline. In other places, DSLs are expected to become more productive than they currently are thanks to the increase in water temperatures fueling more life. Overall, researchers predict that they’ll be more patchy than they are today.

DSLs are not the Phantom Bottoms they were once thought to be. They’re essential ocean ecosystems and a part of the global carbon cycle. As scientists continue to study DSLs around the world, it’s becoming increasingly clear that they’re changing before our eyes.

And as they transform, for better or worse, the ocean’s ability to trap carbon will change along with them. One can only hope that DSLs won’t disappear entirely, becoming another kind of phantom altogether. Phantoms may disappear, but when you use Linode to back up your files, you can be sure that those documents won’t.

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Thank you to Linode for supporting this SciShow video! [♪ OUTRO]