It's the least explored part of the ocean.

A lot of us are familiar with the idea that there's lots of things living in the upper ocean, but we have maybe forgotten about the deep ocean and don't realize how interesting it is and how important it is to our planet.

The creatures here live in the dark at pressures that would kill us.

And far from the ocean floor, they're constantly moving, making them harder to track.

You go down in the deep water and that is just paradise for those animals.

There're so many different kinds down there that nobody has ever seen before.

Scientists from the Woods Hole Oceanographic Institution are embarking on an unprecedented six year project to explore the twilight zone and what they're finding could be key to feeding and protecting life on Earth.

I'm Kayle Hope, and this is Quartz.

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This is the second expedition for the ocean twilight zone project.

By 2025, they expect to make dozens of trips to study the deep sea all over the world.

The twilight zone is home to more fish than any other part of the ocean.

Pretty exciting actually to be able to go down in a submarine and look out the window or the port.

And you see some animal that you know probably nobody in the history of humanity has ever seen before.

And especially even better if you can catch it and bring it back and describe it and give it a name.

There are only 15 or 20 submersibles in the world that can go this deep.

The twilight zone is kind of this important piece of the puzzle that's connecting the surface ocean and the deep ocean in intricate ways.

Heidi Sosik is the lead scientist on the project, and the sub is just a small part of their research.

They're towing a system of nets behind the ship that can be open and closed with a click of a mouse at different depths.

They're also collecting DNA from the water to better understand what organisms live there that we can't see.

We can identify possibly new organisms that don't exist in any reference database yet.

And they want to understand more about where all these creatures are going and what they do.

We're 300 meters down in the twilight zone, looking to see what we can find.

Most of the animals that live in the twilight zone are quite small, but they're able to make these incredible movements through the ocean.

This is called the diel vertical migration.

It's the largest migration of animals on earth and it happens every day.

The animals in the twilight zone don't just stay there.

A lot of them move through the ocean hundreds of meters every day up into surface waters.

When it's dark at night, they come up and they can be kind of stealthy and feed on plankton that had been growing in the sunlit surface ocean.

So there's this like sweeping wave of animals moving up and down through the ocean every day, all across the globe.

They're part of what's called the global carbon pump.

Tiny plants and bacteria at the surface pull carbon dioxide out of the air using photosynthesis.

During the diel migration, deep sea animals come up to eat them.

Then they descend, pulling carbon down with them and adding to what's called marine snow.

Each of those particles is carbon.

And it's heading down to the sea floor.

And if it makes it all the way down there, it can be stored away for thousands of years.

Joel Llopiz is a fish and zooplankton ecologist.

He's also a co-chief scientist on this project.

He says understanding the carbon pump could be key to fighting climate change.

And it's one more thing scientists are just starting to understand about the twilight zone.

The truth about the twilight zone is that we don't know what we don't know about it.

There could be a whole lot of species down there that we have not discovered yet.

And he says, scientists are in a race against time.

The animals here could become a big food source for aquaculture.

The world's population is growing so fast, and humans are struggling to feed themselves.

The twilight zone is a rich source of protein that could not feed humans directly, but it's likely to be fishmeal for food that we do eat.

The scientists from Woods Hole want to gather as much information about the twilight zone as they can before humans change it irreversibly.

I think the challenge for us as humans who care about the health of our planet is that we know so little about the life in the ocean twilight zone that we don't have the information that we need.

We don't even know how long many of the fish that live there live.

We don't know how long it takes them to reproduce.

And without that kind of information, you're flying blind.