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The Earth is mainly a water world — more than 70 percent of its surface is covered by oceans — and yet we know so little about what resides beneath the waves.

The ocean, in this light, is like an alien world within our own. Many of its creatures are still unknown to us — both in kind and number. Their behaviors and adaptations remain inexplicable. Even the very contours of this world are still unmapped: We probably know more about the surface of Mars than we know about the ocean floor.

Understanding the sea is to understand our planet better, at a fundamental level. “There’s so much about how the planet works that is basically preserved in this sort of underwater museum,” Vicki Ferrini, senior research scientist at Columbia University, told Vox’s Mandy Nguyen in 2021.

But not only is the ocean a source of mystery, it’s also a place of adventure. On Unexplainable — Vox’s podcast about big mysteries and all the things we learn by investigating the unknown — we’ve been talking to the scientists who have gone on journeys to understand this watery realm. They’ve come across fearsome creatures like the giant squid, conducted forensic investigations of mysterious deaths, visited octopus cities, and ventured down as deep as humanly possible.

“How could you not be excited about it?” Ferrini said of ocean exploration. “People have this passionate enthusiasm for outer space, which is totally understandable. But the ocean is equally if not more exciting to me because it’s here. It’s the same planet that we’re on.”

And there are countless discoveries yet to make in it. So let’s dive in! Here are seven great ocean mysteries we’ve uncovered, starting at the surface, and then going down deep, deep into the dark.

Where did Earth’s water come from?

Alexander Gerst/ESA via Getty Images

Before we dive into the ocean, there are questions about why Earth has them to begin with. Really: Scientists don’t fully understand how water came to cover two-thirds of the surface of our world.

The problem is simple. When the Earth was forming, it was extremely hot. Any water that was around at the beginning would have boiled away.

“So how do you get so much liquid condensing onto the surface of a planet that should be really, really hot?” Lydia Hallis, a planetary scientist at the University of Glasgow, tells Unexplainable’s Noam Hassenfeld.

Scientists can think of a few plausible options. Was it delivered by comets crashing into our world? Or more fantastically, do we only have water due to the extremely circumstantial event of planets like Jupiter wandering toward the sun from the outer solar system? Or was it, somehow, deeply buried within the early Earth?

The possibilities matter because they can help us understand why there is life on Earth. Without water arriving on Earth, life as we know it would not exist.

Hallis has been traveling the world to investigate and try to find some samples of the very oldest water on Earth.

Here’s what she’s learned so far:

Where is plastic pollution in the ocean hiding, and how does it get there?

A plastic bag floating on the sea off the coast of France. Fred Tanneau/AFP via Getty Images

Now, an ocean story that starts on the surface, on land.

Every year, tons of plastic manufactured on land is dumped into the sea. But scientists have yet to complete the work of figuring out all the nooks and crannies our plastic pollution nudges its way into.

“Ninety-nine percent of all the plastic is missing,” oceanographer Erik van Sebille says on Unexplainable. “We have dark plastic. Like the astronomers have dark matter and dark energy, we oceanographers, we don’t have an idea where most of the plastic in our ocean is. We’ve lost it.”

Researchers like van Sebille want to know where the plastic is going so they can better understand its effects on marine life. What harm is this plastic causing to marine life, and can it be undone?

What powers rogue waves?

“Under the Wave off Kanagawa” by Hokusai VCG Wilson/Corbis via Getty Images

At the surface of the ocean, rogue waves can seem to come out of nowhere. These walls of water are at least twice the size of the biggest waves around them, sometimes towering over 100 feet. Unlike tsunamis, they are not set off by an obvious event like an earthquake. And for a long time, scientists weren’t even sure rogue waves existed; perhaps they were just a convenient myth to explain the mysterious disappearances of many ships over the years.

But all that changed in 1995 when one was recorded for the first time.

Since then, scientists have been trying to figure them out. How do these giant waves form, and what sustains their towering height? “Anything out of the ordinary deserves scientists’ attention,” Ton van den Bremer, who studies fluid mechanics at Delft University of Technology in the Netherlands, tells Unexplainable’s Meradith Hoddinott.

By probing the extremes of rogue waves, scientists like van den Bremer are hoping to gain a deeper understanding of the physics underlying many types of waves.

Why do whales strand themselves on beaches? And are humans to blame?

Rescuers try to help a sperm whale stranded on a beach on April 19, 2022, in China. VCG via Getty Images

Diving deeper, we find marine life. Scientists are constantly asking questions about their most peculiar behaviors.

For example, every year, thousands of marine mammals like whales end up trapped on beaches or in the shallow waters near shore. According to some studies, these strandings have been increasing.

But why do the animals do this? And are humans to blame?

It’s an incredibly difficult question to answer because while we know that humans are affecting the ocean environment, it can be hard to parse how those effects impact individual species.

But it’s important to figure it out. Because how do you protect animals when you’re not sure exactly how you’re harming them?

Enter Darlene Ketten, (a.k.a. “Dr. Doom”), a Woods Hole researcher who specializes in forensic investigations into whale deaths. She conducts Law and Order-type investigations into whale deaths. “What keeps us in the lab and on the beach is looking for more and more information,” Ketten says on Unexplainable, where she explains how she conducts her investigations.

Can a human really be friends with an octopus?

Close-up of a common octopus (Octopus vulgaris). A. Martin UW Photography/Getty Images

In 2020, the documentary My Octopus Teacher provoked a fascinating question: Can a caring relationship form between humans and a sea creature like an octopus? It’s not known whether the friendship in the documentary was genuine from the octopus’s perspective. The interior lives of animals may never be fully understood.

But it’s a fascinating question to think through. “It’s like interstellar travel,” science writer Ferris Jabr says on Unexplainable. “It’s like the closest we can come to that kind of alien contact moment.”

If we can connect with an octopus, what else could we connect with?

How many fish live in the ocean’s mysterious “twilight zone”?

Amanda Northrop/Vox

As you dive deeper into the ocean, less and less sunlight shines through. About 200 meters beneath the surface, you reach an area called the mesopelagic, or the “twilight zone.” Sunlight fades almost completely out of view, and our knowledge about these dark depths fades too.

“It’s almost easier to define it by what we don’t know than what we do know,” Andone Lavery, an acoustician at the Woods Hole Oceanographic Institution, told Vox’s Byrd Pinkerton. “It’s remote. It’s deep. It’s dark. It’s elusive. It’s temperamental.”

Yet this region of the ocean is extremely important. It’s possible — but not certain — that more fish are living in the twilight zone than the rest of the ocean combined, and these creatures of the dark ocean play a large role in regulating the climate.

Why do so many sea creatures glow?

It’s wrong to say there’s no light in the depths of the ocean. There’s light, it just doesn’t come from the sun. Deep in the ocean (and also on the surface), divers find other-worldly displays of bioluminescence, sparkling like fireworks in the dark. Almost every deep-water creature lights up in some way.

“There was just all of this flashing and glowing and sparkle all around me,” marine biologist Edie Widder recounts on Unexplainable about her undersea adventures. “You’re not viewing it at a distance. You’re in the center of the display. In fact, you’re part of it because any movement you make triggers flashes all around you.”

Widder has spent her career trying to figure out why so many marine creatures glow. The quest has led her to confront some of the weirdest, most elusive creatures on Earth.

Only 20 percent of the ocean floor has been mapped. What’s down there?

Currently, only 20 percent of the seafloor has been mapped, making it a more mysterious place than the surface of the moon or Mars. That means every time explorers go down to the bottom, they are potentially seeing things no human has ever laid eyes on before. More people went on the Apollo missions to the moon than have been to the Challenger Deep, the deepest part of the deepest trench in the ocean.

Nicole Yamase is one of those lucky explorers. She told Unexplainable what it’s like down there.

What secrets are buried in the mud at the bottom of the ocean?

Scientists’ curiosity doesn’t just stop at the bottom of the sea. They’re also interested in what lies beneath it.

Sixty years ago, geologists tried to drill down through the seabed to pull up a piece of the Earth’s mantle, a deep layer of the Earth no human has directly observed. Their mission didn’t go exactly as planned. But it sowed the seeds for a new field of science that has helped rewrite not only the history of the planet but, potentially, our definitions of life itself.

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