We filmed this episode quite a while ago and as we were digging into the production, our fellow YouTuber Cleo Abram posted one on the same subject– which is great and you should watch it.

But it turns out her adventure to the center of the planet turned  out to be quite different from our own, which I think is very interesting. So, we decided to use the  video we had already shot and tell our own story.

As a kid, did you, like me, have that dream of just digging a hole at the beach and then making it all the way to the center of the Earth? My friend Jeff dug a hole in his backyard until he hit the water table. It was a well!

But as always happens, our dreams get cut short long before you reach bedrock. But over the years, people have kept at it. Scientists and engineers all over the world have tried digging as deep as possible, hoping to learn more about  the inside of our planet.

Despite their best efforts  and decades of drilling, these projects have barely scratched the surface. Turns out, nearly 6,400 kilometers of planet might be a bit too much of a  challenge for us right now. But what if we could do it?  What if we had the tech, like in the terrible-brilliant  2000s film The Core, to blast all the way to the middle?

What would we experience along the way? I’m going to take you on a journey  to the center of the Earth. On the way, we will discover  huge fields of diamonds, solid rock flowing almost like liquid, and a mysterious inner inner core that scientists are only just beginning to understand.

You might want to grab a glass  of water. It’s going to get hot. [♪INTRO] The first stage of our journey  takes us through the crust. And if your objective is to  get through the top layer of the planet as fast as possible, we might start in maybe an unexpected place: the ocean.

Oceanic crust is the thinnest kind on Earth, maxing out at about 10 kilometers in depth, made pretty much entirely  out of basaltic igneous rock. In the 1960s, the US chose  this seemingly easy route, with their ‘Project Mohole’  aiming to get samples from the oceanic crust off the  coast of Guadalupe, Mexico. The problem is, before you  even get to that igneous rock, you have several kilometers of ocean to deal with.

Geologists have described  drilling into the ocean floor through the ocean as being like drilling through the ground from a helicopter. It was difficult and expensive,  and Project Mohole only managed to dig a hole about 180 meters deep before the money and everyone’s patience ran out. Even with today’s technology,  honed by decades of deep ocean drilling, researchers  have only penetrated a little more than 2 kilometers below  the sea floor, less than a third of the distance needed  to make it through the crust.

Which leaves us with starting  our hole on dry land, on a nice stable bit of continental crust. The kicker is that this crust  is a lot thicker than the stuff beneath the oceans, and we could be facing up to 70 kilometers of rock on  our crustal journey alone. And as we delve deeper into these rocks, the conditions make our project a bit tricky.

Temperatures increase by 25  degrees Celsius for every kilometer we descend towards the  planet’s superheated interior. And thanks to the weight of  rocks above, the pressure increases by 1 atmosphere for every 3 meters down. Ultimately, this is what has  stalled all of our real-world attempts to dig to the center of the Earth, like the Kola Superdeep Borehole - the USSR’s answer to Project Mohole.

Yes, in addition to having a space race up where we went up, we had an inside race, too, where we tried to go down. And the USSR beat us on that one. We’ve talked about this  amazing feat of engineering on Scishow before, and it  still holds the record for the deepest hole on Earth,  at just over 12 kilometers.

But despite more than two decades of drilling, this distance is barely a fifth of the way through the continental crust, and 0.2%  of the way through the Earth. The reality is, any real-world  drilling effort will ultimately falter because at some point,  things will get so hot that the metal in the drills will  become too soft for them to work. There are some people working on giant lasers to do this though, so that's cool.

There is a reason we know more  about the furthest reaches of the solar system than we do  about the inside of our planet! There's basically nothing between us and Pluto. Like, it's just a vacuum the whole way!

It's the opposite when you start going down. But remember, we’re suspending  our disbelief here and conveniently setting aside silly  little things like material physics. Let’s just say we could do  what the Soviets did, and more… The science of seismology— which uses earthquake waves as a kind of sonar for Earth’s interior— can give us a good idea about  what we might encounter.

At depths of more than  three and a half kilometers, we will still find animals,  in the form of devil worms, which are cool as cucumbers  despite the heat and pressure. Those worms eat microscopic bacteria that live down there in the rocks too. And those simple lifeforms can  withstand even more extreme conditions, surviving more than  5 kilometers deep into the crust.

Of course, all life needs water to survive, and tiny cracks from the  surface can allow fresh water to permeate as deep as 10 kilometers down. But there are also pockets of  salty water down there too, trapped by the surrounding  rock. With no way to circulate, scientists think it could have been sitting there for many millions of years and could contain ancient microbes dating from the geological past.

And we’ll find ancient fossilized life here, too All fossils were once living  organisms on the Earth’s surface– or they're products of living organisms, you can have fossilized poop– but over time, geological  processes and plate tectonics push them down, deep into the crust. The Kola borehole revealed  fossils of single-celled organisms 7 kilometers down, dating  from 2 billion years ago. So our journey to the center of the Earth could take us on a journey back through time as well.

However, the deeper we go  through the continental crust, the less likely we are to find fossils. High temperatures and pressures reshape minerals to make metamorphic rocks  with interlocking crystals that are some of the hardest in the world. Limestones become marble, mudstones become schists and gneisses.

And in some places, the rocks themselves melt and recrystallize as huge blobs of igneous rock. After tens of kilometers of digging, we will finally reach the mantle. The boundary between the  crust and the mantle is known as the mohorovicic discontinuity,  or the ‘moho’ for short.

It’s a place where seismic waves suddenly speed up as they pass into a rock of a new composition. Before we hit the moho, the walls of our hole would be lined with huge black crystals of gabbro. Which frankly, would make some  absolutely delightful countertops.

But the view changes once  we pass into mantle rocks. Here, our drill hits peridotite, which  is rich in glittery green olivine crystals with scattered red garnets  like raisins in a fruit loaf. I bet you never read a geology  textbook that told you that the mantle was green with red  polka dots, but that’s pretty much going to be our view for  the next 2,800 kilometers!

Once we’ve passed through the moho, we’re officially in the mantle. But we’re still slogging through the hard outer shell of the planet known as the lithosphere. The reason the crust and the  topmost section of the mantle are grouped together is  because they stick together.

Even though average temperatures are around 500 degrees Celsius, both  crustal gabbro and mantle peridotite are still solid and  rigid, so they behave as one. This lithosphere extends 150  kilometers beneath the oceans, and 300 kilometers beneath the oldest continents. But our trip through the  lithospheric mantle is far from dull.

Because here, scattered in amongst  the green and red peridotite, we can expect to find bright  flashes from a layer of diamonds. In fact, this is where most of the natural diamonds on Earth are born. Volcanic fluids are shifted  around by plate tectonics, and those fluids can contain  all kinds of elements, including carbon.

When those carbon-rich liquids are shunted towards the bottom of the lithosphere,  the high pressures and temperatures there are just right  for building diamond crystals. Violent volcanic events have  brought some of those diamonds to the surface, in places like  Russia, Botswana and South Africa. But there’s a far bigger reservoir of the gems lurking at the bottom of the lithosphere.

We can enjoy a few hundred  kilometers of breathtaking, crystalline scenery as we  travel through the lithosphere. But when the increasing temperatures  hit 1,300 degrees Celsius, everything changes as we enter the asthenosphere. At this temperature, it’s hot enough for the mantle rocks to start behaving strangely.

Now, you might be expecting to hit something like molten magma at this point. After all, we learn that it’s the convection of the mantle that powers the  movement of the continental plates. And while the convection part is likely true, I’m sorry to say that hitting the mantle isn’t going to be like striking oil,  creating a fountain of lava.

Nope, the upper mantle is a solid. But because it’s so hot, it  flows almost like a liquid, albeit really really slowly. In this case, the mantle convection  currents involve solid rock moving at about the same speed  that your fingernails grow.

So, as far as we would see as  we drilled down through it, the upper mantle is still  that glittery green solid, encrusted with garnets. Come back in a hundred years though, and your hole wouldn’t be quite where you left it! At a depth of about 410 kilometers, the behavior and composition of  the mantle changes once again.

Between here and a depth of 660 kilometers, we’re passing through the mantle transition zone, which is marked by two changes in seismic wave speeds at the top and bottom. Scientists aren’t exactly sure  what’s causing these shifts, but they think that it could be thanks to a huge reservoir of water trapped there. It’s likely that there’s  about three times the amount of water down here as in all  the oceans on the surface.

But before you get carried  away imagining a sloshing shell of liquid down here, this is  not, like, a subterranean ocean. Instead, it’s water that’s  bound up inside crystals of a mineral called ringwoodite,  which only forms at the precise temperatures and pressures found at these depths. So the next time you see someone on Instagram saying, "There's a giant subterranean ocean!" You can be like, "No!

Ringwoodite!" Like what I do! Every time! The water in ringwoodite would  make the crystals bright blue.

We might not be able to dip  our toes in the mantle’s ocean, but we can enjoy 250 kilometers of beautiful blue rock as we descend nonetheless. Just don't touch it! Don't touch any of this!

At the end of the mantle transition zone, ringwoodite is no longer  stable, and our drill tip might begin to struggle a bit as  it enters the lower mantle. Here, things are hotter still, averaging around 3000 degrees Celsius. But the pressure of nearly 700 kilometers of overlying rock is enough to  keep the mantle solid and stiff.

Unlike the asthenosphere above, some geologists think that the lower mantle doesn’t convect. But slow-downs in seismic waves seems to suggest that there is still  some structure down there. Specifically, there are two  massive blobs sitting opposite one another like earmuffs on the outer core, roughly under Africa and the north Pacific.

And when I say massive, I mean it - each one is twice the size of the moon. Scientists think the seismic waves slow down here because the blobs are iron-rich, but there’s some disagreement about where  they might have come from. Some speculate that they’re domes of extra-hot material that’s rising up from the core and interacting with the colder mantle around it.

But there’s another more intriguing possibility: that the blobs are the  remnants of a cataclysmic event in Earth’s past, like the  planetary impact that created the moon about 4.5 billion years ago! That could leave chunks of very different rock buried deep in the mantle. If relics of Earth’s ancient  history weren’t enough for you, as we continue our journey down towards the bottom of the lower mantle, we will find ourselves in yet another reservoir of diamonds.   They form thanks to interactions  at the mantle’s lower boundary, as water molecules split apart  and react with iron carbide.

This kicks carbon out of the core, where it’s forged into diamonds under extreme pressure and temperature. Since they have more than 2,800 kilometers of rock above them, most of these core-mantle diamonds will never make it to the surface. But some scientists believe that the famous Hope Diamond may have come from the deep mantle.

After drilling down for 2,890  kilometers, we’ve finally made it through the mantle,  and into the outer core. The temperature has risen gradually to nearly 4,000 degrees Celsius at the core-mantle boundary. And for the first time, our drill tip is biting into a completely new kind of material.

Instead of rock, we’re now drilling through metal. Specifically, an alloy of iron and nickel, with some other elements such  as gold, platinum, and cobalt. Because of the high temperatures, this metal mix exists in a liquid state.

Its complex motion is affected  by the planet’s rotation, and turbulence in the outer core is responsible for generating the Earth’s magnetic field. Which I am a huge fan of! It's great!

However, it’s not exactly  something you could go swimming in, and not just because it would sear your skin off. Because that liquid is thick. High pressures at this depth  mean that even at the top, the molten metal is about the  same consistency as peanut butter.

Which I have never tried to go swimming in, but don't think I would be good at, and it gets thicker, like thicker  than tar towards the bottom. So while the outer core’s motion is super significant on geological timescales, we wouldn’t exactly be swept  away as we passed through. By the time our hole is 5,150 kilometers deep, we officially enter the inner core.

Here, the temperature is  around 5,500 degrees Celsius, similar to what you might find  at the surface of the sun. And the pressure, which has been  increasing all the way down, is now 3.6 million times that at the surface. The material we've been  drilling through for the last few thousand kilometers hasn’t changed, but despite the extra heat, the iron-nickel alloy is no longer a liquid.

Instead, the intense pressure is enough to force the metal back into a solid. Curiously, the solid metal that forms under these conditions is unlike anything  you would find at the surface. It exists as extremely tightly  packed hexagonal iron crystals that are all aligned north-south  by the Earth’s magnetic field.

I like to imagine it looks  like a metallic version of the giant’s causeway in Ireland, or  the Devil’s tower in Wyoming. There are a little more than 1,200 kilometers of these close-packed iron crystals before we officially reach the center of the Earth. But there’s one more surprise  waiting for us on our way.

Inside the inner core, at a  depth of some 5,720 kilometers, there is an inner, inner core,  like a pit inside a piece of fruit, which scientists have only recently discovered and still know very little about. First theorized in 2002, its  existence was confirmed in 2019 thanks to seismic waves from  extremely strong earthquakes. Researchers realized that seismic  waves were traveling at very different speeds depending on  which direction they were going through this part of the inner  core, which is thought to be thanks to another difference in crystal structure.

One possibility is that the  iron-nickel crystals switch to being aligned east-west instead of north-south, as if our metallic Devil’s Tower  had been turned on its side. What’s even less clear is why  there’s a 650 kilometer thick ball of crystals on their side  in the center of our planet. It could be a fossilized  record of some major event in Earth’s past, but at that  depth, we may never know.

So here we are. After 6,371  kilometers of drilling, we’ve made it to the center of Earth. We’ve traveled through the  solid rock of the lithosphere, solid flowing rock of the mantle,  liquid metal of the outer core, and crystalline metal of the  inner and inner-inner core.

If we kept on digging now, we’d  get to see it all again in reverse, with the added challenge of  gravity working against us! It took the Soviets 24 years to drill through 12 kilometers of crust for  the Kola superdeep borehole. Even if we managed to match their pace despite soaring heat and pressure,  at 2 years per kilometer it’d take us nearly 13,000 years to make it all the way through the planet.

So if you’re on the beach and serious about making the trip, better start digging! [♪OUTRO]