The tallest mountains on Earth are ... underground?

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Samantha Hansen has a comic on her office door where a scientist says this: I live next to a wall of rock 20 miles thick.

There's no way around it or over it.

I'm trapped on this side forever.

I study the stuff on the other side.

And that pretty much sums up my life, right?

Like, I live on one side of a wall that I can never get across, and I study the stuff on the other side.

Samantha's a geologist who studies the interior of the Earth, miles and miles below us.

We live on this planet that takes care of us and provides this home for us, but yet we have little to no understanding of how much of it works.

And she can never study it directly.

There's no drilling 100 miles deep, so she needs to travel the globe, probing the ground with various instruments.

That's how she ended up in Antarctica in 2012.

At that point, no scientists had studied deeper than three miles below its icy surface.

Antarctica's beautiful in a very stark sort of a way, but I have never been anywhere on the planet where I've seen so much nothing.

There are times when you're out in the field that you cannot tell where the ground ends and the sky begins.

And as Samantha and her team used their field equipment to peer underground, they found something huge.

Something hidden under all of our feet.

Something that could help us better understand some of Earth's biggest geological processes.

Things like tectonic plate movement, why we have a magnetic field, or why we're even here at all.

Ultimately, a planet's ability to support living organisms has a lot to do with how that planet functions.

Why is Earth the only planet that we know of that has life?

And how do the processes that drive this planet ultimately sustain that life?

I'm Manning Went, and this week on Unexplainable, what Samantha's team found under Antarctica and how their discovery shakes up some of our most basic understandings of the earth.

As you might imagine, working in Antarctica is a little challenging.

I grew up in Wisconsin, so I'm used to winter, but not surprisingly, it's cold.

Samantha and her team spent six weeks stationed at a research center in Antarctica.

Antarctica.

And even though it was the middle of summer, the conditions could be brutal.

The wind was howling and you were up at high altitude and your working conditions were somewhere in the neighborhood of about minus 40 degrees.

Awfully chilly.

But when the sun was out, the wind was calmer, and it was only about freezing, those were the days that Samantha could get out to her work site.

You drive out to the airfield with all your equipment and whatnot, and you board a very small airplane, probably could park it in my living room.

The first thing Samantha did at the site was grab a shovel and start digging.

My biggest thing was also keeping my hands warm and keeping my fingers doing what they needed to do.

It took Samantha's team a couple hours to dig a few waist-deep holes.

Then they had to lower down hundreds of pounds of equipment.

Basically, those instruments are what are recording the signals from earthquakes.

These instruments are recording earthquakes that are happening all over the world, not just in Antarctica.

Anything above about a magnitude five, anywhere in the world, we can see that.

And those earthquakes help Samantha understand what's happening inside the Earth.

When an earthquake happens somewhere, it sends energy through the inside of our Earth.

The energy she's talking about is a series of seismic waves, which are basically huge vibrations.

They can come from earthquakes, but they can also come from things like volcanic eruptions or landslides, anything big big enough to shake the earth.

And as those waves move underground, they get altered and distorted.

What's inside this planet has a big impact on those waves.

You know, how fast these waves move depends on what it's moving through.

And that's sort of the heart and soul of seismology.

It works a lot like sound, which is also just waves of energy.

Listening to someone's voice sounds different depending on whether you're hearing them through a concrete wall,

whether you're hearing them through sand,

whether you're hearing them underwater.

This is how Samantha can study the deep earth underneath Antarctica and see beyond a wall that she can't cross.

Scientists have been doing these types of studies for decades, all over the world, and they've learned the basics of what the interior of our planet looks like.

At its simplest, you can think of the Earth like a big layer cake.

The frosting, or the very top layer, is the crust.

So the crust is kind of the thin outer skin of the planet that we live on.

But the crust is only 1% of the Earth's volume.

There's another layer below it that's 60 times thicker called the mantle.

The mantle is kind of soft and squishy.

It's not molten, though it's not liquid.

Below that, there's the core.

the center of the Earth.

Which is comprised mostly of heavy metals like nickel and iron.

And it's in between the layer of the mantle and the core that Samantha found something really strange.

When seismic waves move through these features, they dramatically slow down.

There were some weird areas down there made of different stuff from the rest of the mantle, and they were distorting the seismic waves.

These areas are called ultra-low velocity zones.

These structures, they're not huge.

they're sort of tens of kilometers wide, kind of tens of kilometers thick, but they have this such different characteristics than the material around them, so they tend to stand out.

They almost look like underground mountain ranges.

It's just the material surrounding the mountains, instead of being air, it just happens to be the Earth's mantle.

We don't know what they're made of.

We just know that they're sort of shaped like mountains surrounded by superheated rock, which is the consistency consistency of tar.

They can range anywhere from three to 25 miles high.

One of the tallest they found is five times taller than Mount Everest.

It's like the Himalayas but jacked up.

Before Samantha did her research, these underground mountains had been discovered in a few limited places.

But this discovery under Antarctica pointed to something larger.

It was almost like everywhere we looked, it's like, and here's another one, and here's another one.

You know, like it sort of brought up this question of like, well, are these things everywhere?

Samantha thinks these underground mountains might form a sort of blanket wrapping around the core of the Earth.

It would basically imply that you have a whole other layer inside our planet.

A new layer to fit in this classic Earth layer cake analogy.

A new layer we don't understand.

But it's only a hunch.

Scientists are just beginning to understand what these mountains even are.

You know, where do they come from?

What's their origin?

What's their source?

There's just so many unknowns.

A deep dive on underground mountains after the break.

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We're inside a giant jump bubble,

wrapped in a cobalt cocoon,

700 miles below the surface of the Earth.

Hello a day.

Scientists have a few ideas about what these underground mountains are and where they come from.

The first has to do with temperature.

The mantle material can partially melt, so you're getting little pockets of molten material.

Waves move slower through liquid, and since seismic waves slow down when they pass through these underground mountains, that might mean that these mountains are liquidy.

So in this case, maybe they're less like literal mountains and more like molten lakes.

Little piles of melt, if you will.

Another idea is that a chemical reaction is happening deep in the mantle.

And water down there might be interacting with iron iron in the core.

So if you have some kind of chemical reaction happening there, that could make, you know, unique property material.

But Samantha is leaning towards a third option.

Maybe these mountains might be pieces of ancient oceanic sea floor that have sunk deep into the earth.

Over, you know, hundreds of millions of years, you're taking, you know, material at the surface and having it dive back into the planet.

This might have happened when tectonic plates, these huge chunks of crust and mantle, hit each other and one slid under the other.

It's a process called subduction.

So your subducted materials were once pieces of the ocean floor that have journeyed all the way down to the core.

One of Samantha's collaborators modeled the movement and subduction of tectonic plates and what happens in the deep mantle.

And what he showed was that over hundreds of millions of years, you end up with this fairly continuous blanket of this material along the Cormanno boundary.

It's not a perfect model.

It's going to take more time and research to nail down where these mountains come from.

But I was also curious about the implications here.

So yeah, if subducted materials are, quote, the right answer, then it's interesting because

it illustrates how the Earth functions as one giant system, top to bottom.

Things happening at the surface are affecting what's happening very deep in the Earth, and vice versa, right?

Some places new stuff is getting created and coming out, and some places it's diving back inside and

disappearing and getting recycled.

So it's like Earth's basically one big recycling system where the whole planet is sort of interacting with other parts of itself.

Why is it important to note where they come from?

That's a fair question.

Part of,

i mean we have so limited understanding of the processes inside our planet how the earth has evolved through time and ultimately that has implications for understanding how our magnetic field is generated which protects us from solar radiation so we can actually live on this planet wait you said we don't know how the earth's evolved well i mean from a very general perspective we do but the details are fuzzy right um so

for example, we know we have a magnetic field, right?

We have a fairly good idea essentially how it's created and why it's here,

but how different parts of the planet sort of work together to make that field, you know, again, the details are fuzzy.

And so getting a better handle on these kinds of things can help us learn a lot about how our world works and where it's potentially going in the future.

Yes.

And we live here.

This planet supports us.

And so if you don't understand how the planet functions, you're not going to understand how life is supported.

What if we don't find out what the answer is?

So yeah, I don't want to disappoint you, but I don't know if I can ever definitively answer this question

unless you know Hollywood becomes reality and we can actually take a trip down to the deep mantle.

It is sort of the inherent nature of the type of work that I do

that we can likely not definitively prove this because the only way you can do that is to physically go sample it or see it.

And

at least at this point in time, that's just not realistic.

We can't do it.

So we make our best estimates using the data that we can.

And that's sort of at the heart of this kind of science.

Right.

Like that's the edge of the science.

It is.

I mean, we are kind of pushing the frontier on this.

And I guess to be fair, you know, plate tectonics is a concept.

That whole idea only got developed in the 1960s, which, I mean, that's my parents' generation.

So earth science is a fairly young science, right?

Especially seismology.

And so I am cautiously optimistic that we will have a good explanation for this at some point.

We're not quite there yet, but I think we're making good headway.

If we do figure out the source of these underground mountains, what other big questions would that open up?

I think it opens up kind of

an area of further study because there are other things down there that we could investigate.

And I think this kind of helps open the door for some of those other kinds of investigations too.

There are other things down there.

Yeah.

That seems like a very big statement.

Like, oh my God, really?

Yeah.

Well, I mean, you know, the aliens aren't coming or anything.

But

so, for example, there are two

appropriately named large, low, shear velocity provinces, which are big and slow that exist on either side of the planet.

One's beneath Africa, the other one's beneath Pacific.

and characterized again by seismic velocities that move much slower through them.

But they're huge.

I mean, the footprint of one of these things is about the size of a continent.

They extend upwards of, you know, a thousand kilometers or more, giant features in the Earth's mantle.

What's causing those, right?

There are also what they just sort of generically refer to as seismic scatterers, little blobs of stuff in the deep mantle that cause seismic energy.

When they run into it, it kind of scatters all over the place, like when light hits a prism.

Wow.

What's causing that?

Are all these things related?

Are they all part of the same process or not?

So there's a lot more to be investigated in the deep earth.

And there's so much variability in that part of the planet that is usually just glossed over when people talk about Earth structure.

But I think people are starting to recognize how important that those details are.

And I think there's a lot of cool room for discovery.

So this is just one of those things.

indeed.

So, we have to get rid of this layer cake analogy then.

It sounds like it may not, yeah.

Well, I mean, we need to stop selling this layer cake model to people because it's probably not really that accurate.

You know, there's a lot of other stuff in there that doesn't totally fit with that model anymore.

But, what point do we stop oversimplifying and you know, start getting into the details?

Do you have a...

Do you feel like there's a better analogy here?

What's the updated analogy?

I don't know if I completely have one yet.

I'm still working on that.

I think a lot of us are.

This episode was produced by me, Manning Winn.

We had editing from Brian Resnick with help from Jorge Just, Noam Hassenfeld, and Meredith Hodenott, who also manages our team.

Mixing and sound design from Christian Ayala, music from Noam, and fact-checking from Serena Solon.

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