- When you think of Earth's largest animal migrations, you might picture massive herds making their annual trek across the Serengeti or transcontinental flights painting the sky orange each year.

But Earth's biggest mass migration actually happens every single night and it's underwater.

During World War II, submarine Sonar recorded these strange dense signals rising from the deep as if parts of the ocean floor were moving up and down by as much as 3,000 feet.

The sea floor wasn't moving.

The sonar was actually detecting huge masses of tiny animals known as zooplankton ascending from the depths to the surface every night and returning down again.

Turns out this happens in every ocean, every night and scientists were completely bewildered.

I mean, why do these nearly microscopic plankton make such an incredible daily journey?

Turns out the answer could be linked to phenomena as seemingly unrelated as biological clocks and even climate change.

Hey, smart people, Joe here.

This is the strange story of Earth's largest and most mysterious migration.

(bright upbeat music) - Vertical migration in the ocean is the largest net animal movement on our planet.

It's really remarkable.

I'm Kelly Benoit-Bird, a senior scientist at MBARI, the Monterey Bay Aquarium Research Institute where I use sound to study lives of ocean animals.

- So first, you really need to appreciate how tiny zooplankton are.

Like smaller-than-the-tip-of-a-crayon tiny but the distances they move in the ocean are absolutely immense, for them anyway.

- If we were to scale the migrations to a human, we'd be talking about you doing a 10K twice a day, once to get your breakfast and once to go to bed but you'd have to swim at twice the speed of an Olympic marathon runner.

It's a pretty remarkable endeavor each and every day.

- If you add up all of the vertical migration happening in all the oceans and lakes on earth, scientists estimate 10 billion tons of biomass, 25 times the mass of all humans on earth is racing between the surface and the deep every night.

It's called the diel vertical migration or DVM for short.

But why go to all that trouble?

- Vertical migration is probably one of the most common behaviors that we see in the ocean.

It happens from the smallest animals to some of the largest and the most abundant, in terms of biomass, migrators are typically small fish like bristlemouths and lantern fish that are following the vertical migrations of the zooplankton.

It is a pretty different way of thinking, most often we've thought about plankton, all of the plankton as just wanderers like the Greek word for plankton defines them but they are are capable of making decisions.

- Zooplankton live in the twilight zone.

No, not that twilight zone, though some do look pretty strange.

We're talking about the mesopelagic zone, it's a region of semi-deep water that receives only about 20% of the light that you get up on the surface.

- Well, we know that this vertical movement is a real dance, a balance by these animals to try to get food which is most abundant in the surface waters where photosynthesis lets things grow but while they're trying to avoid becoming dinner for something else And so if you're trying to avoid getting eaten, you wanna be in the dark.

So most often what we see is that animals stay deep in the dark during the day and then as the sun sets, they migrate up to the surface before leaving again at sunrise.

- Responding to tiny changes in light that would prompt them to move up the water column when the sun went down and then back down at sunrise.

- But sometimes we see, organisms actually do the opposite.

We sometimes call reverse diel vertical migrations.

- Researchers found that zooplankton move up and down in the water by as much as 200 feet just from clouds passing overhead.

That means they're pretty dang photosensitive.

But scientists thought there might be more to the picture.

- Just like those early observers of diel vertical migration, we're using sound.

Light doesn't penetrate very far in the ocean so when we try to go down with a camera with a lot of lights, we're lucky if we see a few arm lengths in front of us but sound travels both further and faster in water than it does in air and so we can get a really large scale picture very quickly of what's happening with animals.

So we combined those sonar observations where we send out a short pulse of sound and interpret how it echos off the animals in the habitat with low tech tools like nets but also with new techniques, looking for evidence of the DNA that these animals leave behind in the water column, for example.

We kinda combine all these different lenses to get a complete picture of what's happening far away from what we can ever see when we're sitting on a ship at the surface.

- Thanks to all this high-tech study, scientists have figured out that DVM is fueled by more than just sunlight changes.

For instance, scientists studying zooplankton in the Arctic saw that during the long, dark winter months, zooplankton responded to moonlight instead.

So all of this new information has totally changed the way scientists think about... Well, it's changing what we know about plankton altogether.

Okay, so there's a different class of plankton called phytoplankton.

They're the ones that do photosynthesis and actually give us most of the oxygen we breathe.

- There are some phytoplankton, some photosynthetic plankton that also undergo vertical movements.

These are tiny, microscopic, plant-like organisms that are drifting constantly with the currents but they can control their vertical movement.

They can control buoyancy and some of them actually have swimming organs but typically we see that they're really close to the surface harvesting sunlight to photosynthesize during the day and then moving deep in the water column at night in order to take advantage of higher nutrient levels at deeper depths.

And so they're sort of shifting the balance instead of worrying about food and predators, they're worried about light and nutrients that they need to grow.

- But that's not all.

It turns out that studying DVM could help us unravel our own circadian rhythm, the biological clocks that help our bodies keep time.

The term circadian comes from the Latin phrase circa diem, which translates to around a day.

And that's pretty appropriate, since circadian rhythms control many of our day-to-day behaviors.

And most organisms that live on the earth's surface, including humans, have a system of hormones and parts of our brain, an internal biological clock that helps control everything from sleep to hunger to fertility.

If you're a living thing that lives on land, light or the lack of light triggers the release of chemicals that signal that it's time to do certain things.

Like, if you're a bee, these chemicals might send you out to hunt pollen.

For plants, it might mean moving their flowers in a different direction.

And for humans, when it gets dark, our bodies start producing melatonin, a hormone that helps us relax so we can sleep.

And when it's light, we produce less melatonin which encourages us to stay awake.

So we know a lot about how these rhythms work in land organisms but when it comes to things that live in water, circadian rhythms were kind of a mystery.

Like, do plankton even have them?

Or are they just little robots that follow light like a moth to your porchlight?

Well, in 2017, researchers were studying zooplankton in the lab, they noticed that not only do they move up and down during a normal day-night light cycle, they also found that they migrated even when the lights were off all the time.

This means these specks of almost alien-like ocean life might have a circadian rhythm just like we do.

But by far one of the strangest things we're learning from studying vertical migration is that it might hold one key to tackling humanity's biggest collective challenge; climate change.

- Vertical migration plays a big role in the biological carbon pump.

Organisms that photosynthesize at the surface take carbon dioxide out of the atmosphere.

If that just stays on the surface water, it just gets re-released back into the atmosphere.

But if it gets into the deep sea, it can stay there for thousands of years.

And so by taking that carbon out of the atmosphere and bringing it to the deep sea, it is removed from the effects of that carbon dioxide is having on climate change.

The ocean has done that.

It takes up every year, about 25% of the CO2 that we release is absorbed by the ocean and sequestered.

Vertical migration is a really fast way to do that.

Animals come up to the surface, they eat those phytoplankton that have taken up that carbon dioxide and then they swim down to the deep sea where they release their waste products, where they are eaten, where they die.

- If we disrupt vertical aquatic migrations, we could speed up the collapse of entire ocean food webs and impact everything from our food supply to the climate.

And all that makes studying the diel vertical migration even more critical.

- I think vertical migration is a really important engine for the ocean.

In fact, if there's a fish on your dinner plate, it probably ate something that was vertically migrating.

So not only is it helping the health of the planet but it's providing a really key transfer mechanism for food for humans as well.

I think it's just amazing to think about the world in as many dimensions as the organisms in the ocean have to.

We're so used to being able to use our GPS and know that if we come back to the same place, it's going to look the same while in the ocean, place isn't really a thing, right?

You stay with the water parcel and that gets swept around.

And if you try to stay in one place, the characteristics of that environment would change dramatically through time.

It's really hard to get your mind around in order to start to think like these animals, to understand the world that they live in.

- It seems kind of obvious when you say it out loud but messing with the largest migration on Earth, this colossal thing that happens every single night, we could really screw up the planet.

We might even end up krilling ourselves.

(audience booing) I'm sorry.

Stay curious.