Sage: Are you ready for this? I don't think you're ready for this. 

This is the deepest hole ever dug.  

[Manhole cover pictured] I swear it's under there. 

It's called the Kola Superdeep Borehole. You can find it in the frigid ground on the Northwest top of Russia.

And if you fell in, which is unlikely since the opening is only about 23cm wide, you'd plummet 12.2km before hitting the bottom. 

That's deep. 

The wild part, this hole barely scratches the surface. It doesn’t even go through the top layer of the Earth. 

So, what really lies beneath us?

Hi, I'm Sage. This is Crash Course: Geology. 

[Theme music]

We humans have been wondering about Earth’s insights for a really long time.

In the 17th century, Edmund Halley, of comet fame, proposed the Earth was a hollow shell holding a smaller sphere, like a gargantuan Kinder egg. 

In 1864, author Jules Verne imagined a world within our world full of oceans and prehistoric life forms. 

And in 1928, Arthur Conan Doyle envisioned a sea urchin-like planet with sentient squishy guts fighting people, back when people drilled into it. 

Dwayne here is very glad they were all wrong. Though the Kinder egg one would br pretty neat, huh? 

Earth's crust, our planet's outer rocky shell, is the layer we know best. It's where we live, after all. But takes up just 1.4% of Earth's total volume. We've never journeyed beyond it, but not for lack of trying. 

In fact, before Kola Superdeep Borehole, there was Project Mohole. 

Let's dig a little deeper  

In the late 1950s, a ragtag gang of scientists known as the American Miscellaneous Society hatched an ambitious plan: Drill all the way through the Earth's crust. Why? Because nobody had before!

The group dubbed their plan Project Mohole, after the Mahoro Vich discontinuity, or the boundary between Earth's crust and mantle. 

And in 1961, the team boarded a ship called, and I can't believe I'm saying this, CUSS-1. On board, journalist John Steinbeck — yep, the Grapes of Wrath guy — compared it to an "outhouse standing on a garbage scow."

Here's the thing about trying something nobody has tried yet. Nobody knows how.

Team Mohole needed to figure out not only how to drill through kilometres of rock, but also how to get that drill to the bottom of the ocean where the earth's crust was thinnest. It was not going to be easy or cheap.

So, they started small. They picked a spot off the coast of Mexico and drilled a few hundred feet, and eureka! They dredged up cores of rock and sediment from the seabed, discovering volcanic rock nobody knew was there. 

Steinbeck wrote that everyone aboard had a frantic interest. So much so that the scientists had trouble working around the crowd.

And back on land, US President John F. Kennedy praised the teams remarkable achievement. 

But then came bureaucracy and bickering. Team Mohole disagreed about the next steps. Should they continue with small tests in shallow hole, or go big, crust or bust? 

One geologist warned that the project would be either one of the most rewarding scientific ventures ever carried out or a foolish and unjustifiably expensive fiasco. 

Turns out ur eas neither. Money ran out. Project Mohole faded away.

The dreamers and the schemers behind Project Mohole didn't reach the big goal of drilling all the way through the Earth's crust.

To this day, nobody has. Though folks are still trying, including my pal, Dwayne.

Grab your shovel, buddy. We're digging through the centre of the Earth. 

We're starting here at the crust. It's got all the fossils and fuel ever buried. All the sewer pipes, basements, and doomsday bunkers ever built. All the incrustables ever made. 

Which reminds me, hand me that, would you? [An incrustable sits in front of Dwayne] Fine, then, we don't have to share. 

The crystal is made of solid rocky slabs, mostly composed of oxygen and silicon, along with other elements like aluminum, iron, and calcium.

These skabs, known as tectonic plates, slam into, slide against, and pull apart from one another over time, forming continents mountains, and trenches up top. 

The crust is thickest under mountains, where it can be 70 km deep, about the distance between Time Square and the middle of Long Island, New York.

But oceanic crust forms differently and is much thinner than continental crust. It's typically only 5-10 km thick.

You could basically do a fun run from top to bottom. As we keep digging, we'd eventually make it to the mantle, Earth's dense middle layer.

It generates conviction currents which help drive the churn of tectonic plates and recycling of rocks from the surface. 

The Earth’s mantle is thicc (with two cs). It makes up roughly 84% of the Earth’s volume with a depth of about 2900 km.

That's about the distance between Tume Square and the middle of Colorado. 

What we've covered so far from the surface to about 100km down is called the lithosphere. It's the solid, rocky, relatively cool part of the Earth, which includes the crust and the uppermost part of the mantle. 

But then, we'd enter the asthenosphere, the swath of the mantle where it's so hot, and the pressure is so high, the rocks become stretchy. 

The mantle is mostly made of rocks rich in iron and magnesium. Asheat and pressure increase, the rocks get loosey goosey, ooey gooey, even. The deeper we go, the more the rocks have the texture of Taffy. 

For the first 410 km or so, we'd be in the upper mantle. Notice how it's greenish black. That colour comes from a sparkling mineral called Olivine.

But the colours would start changing as we moved into the mantle transition zone, and area increasing temperature and pressure that causes the chemical composition to shift. 

The deeper you go, the hotter and more densely packed the minerals are, and some can't take the pressure. Their atoms rearrange to become completely different minerals, changing colour to black, red, and blue. 

Just as we'd hit the base of this transition zone around 660 km down, we'd find some wild terrain. A subterranean mountain range with jagged crags that no one has ever climbed, some of which might even be taller than Everest. 

Past the mountains, we'd enter the even hotter, denser, lower mantle, which stretches from 660 km to about 2700 km below Earth's surface.

Here, because of the high pressure, the rocks flow less than they do in the upper mantle. 

And the lower mantle is big. It accounts for about half of Earth's total volume.

But then, what's that? Strange blobs the size of continents.

What are they? We don't know yet. Maybe they're leftovers from an ancient collision with another planet. Or they're bits of tectonic plates that have broken off. 

For now, scientists have nicknamed them Tuzo and Jason. 

I love geologists and their strange naming conventions. 

After that skight detour, we'd arrive at the core, Earth's extremely hot, extremely dense centre. We'll talk about that in our next episode. 

So there you have it. Earth's three layers: crust, mantle, and core. Like one giant peanut M&M.

Even with powerful drilling equipment, researchers still haven't been able to get past the crust. 

So, you might be wondering, how do we even know this stuff? Well, geologists have figured out ways to study Earth's deep interior by finding clues on the surface. 

They can learn about the mantle's composition through volcanic eruptions that spit out melted mantle rock, or rip out hunks of it.

And they get by with a little help from space, too. From iron meteorites, very old space rocks that likely form the core of ancient worlds.

Geologists know Earth's core is made of the same metals which sank to the planet's centre as it formed.

And in 2014, a 4.5 billion year old meteorite made it possible for geologists to observe for the first time the most abundant mineral inside Earth, bridgmanite, which scientists think makes up much of Earth's super hot lower mantle. 

Geologists also study deep earth through seismology, a field that analyses seismic waves from earthquakes and other vibrations within the earth. 

From seismic wave measurements, geologists can take x-rays of the Earth, piecing together properties of its layers. 

And even though we still haven't made it to the mantle, Project Mohole and the Kola Superdeep Borehole weren't failures. They kicked off techniques that later scientists used to discover more about Earth's insides and past. 

Drilling and dredging up samples from the ocean floor has since brought insight to the Earth's climate millions of years ago and shaped our knowledge of how tectonic plates sculpt the landscape over time among many other discoveries. More on that in future episodes.

There's still so much to learn from the inner earth, including how it connects to us. 

In 2023, a team on a ship called JOIDES Resolution drilled about 850 m below the ocean surface and pulled up chunks of mantle from an underwater mountain where the crust is especially thin.

They observed how those rocks, sparkling green olivine, reacted with the ocean's water to create the hydrogen required to form organic compounds, life's basic ingredients. 

Those mantle rocks could offer clues to how life began billions of years ago. 

To this day, nobody has actually journeyed to to the centre of the earth, or even below the crust for that matter. By many measures, we know less about our own planet's insides than about the solar system outside it. 

But geologists have found ways to work around these challenges, building on early drilling techniques and refining new methods for studying the Earth's depths. 

There is so much more to discover about the world beneath our feet and how it shapes our lives up here. 

Next time, we'll put a spotlight on Earth's magnetic MVP, the core. See you then. 

Thanks for watching Crash Course: Geology, which was filmed in our studio in Indianapolis, Indiana, which was made with the help of all these dope people. If you want to help keep Crash Course free for everyone forever, you can join our community on Patreon.