What If You Just Keep Digging?

If you’ve ever thought, “what if I just dug 
a really really deep hole?”, that’s what the USSR did right here! That hole is deeper than the 
deepest part of the ocean. It’s deeper than Mount Everest is tall. They started digging it in the 
1970s as part of basically a space race but down the United States only got to 600 ft before 
pulling funding but the USSR kept going for 20 years. They made it about a third of the way 
through Earth’s crust and then stopped. But what if you just… kept… digging? If you dug a hole all 
the way to the center of the Earth, what would you find? What would happen to you? And what does our 
newest tech tell us is really going on down below our feet? “Let’s take a look at what’s inside!” “to the 
center of the Earth” “went where no human being had ever set foot” “still falling!” “the center of the Earth” 
“the heat and the pressure” “what’s underneath the Earth’s crust is different” “the deeper you go into 
the Earth the hotter it gets” “that’s wild!” Imagine we go outside and we just start digging. 
At about 50 m deep we might start to feel the temperature go up. Every kilometer through
the crust is about 25 C hotter. Right now, we’re inside the crust.
On land, it’s typically less than 40 km deep which is less than the length 
of a marathon. We’re passing through soil and layers of hard rock past precious metals and 
fossils and as we get deeper we start to see all kinds of impressively deep things that 
people have built like there’s the Kamioka and Sanford research Labs that do dark matter and 
neutrino physics research or there’s China’s insanely deep nuclear command bunker. There’s 
the deepest cave we’ve ever discovered plus some of the deepest mines in the world. As we 
go deeper, temperature and pressure continue to rise but by this point, unprotected just in 
the Earth, we were crushed and crispy a while ago. To go farther, let’s equip ourselves with an 
imaginary drilling machine that’s immune to any temperature or pressure. Now finally at 12.2 km 
we reach the deepest point humans have ever dug this is that hole the USSR dug the Kola Superdeep
Borehole. They didn’t put any humans down here this equipment was operated remotely. To 
get so deep, these scientists had to invent new equipment and drilling methods to pierce through 
the thick hot pressurized rock. Along the way they discovered new fossilized organisms dating back 
2 billion years years and found there was water much deeper than scientists previously thought. 
We actually emailed three of the scientists who worked on this project and one of them told us 
“the Russians tried several times to get down past the maximum depth that they had reached 
but each time the New Hole tended to collapse.” This was a “moonshot which will never be repeated or 
surpassed.” This is so cool but the craziest part is this deepest hole that humans have ever dug 
is only .2% of the way to the center. Time to go deeper. At about 30 to 50 km down, we noticed that 
the rock around us is changing. We’re crossing into the mantle. At first the rock around us looks 
really brittle, like the crust but denser. But as we get a little deeper as the temperature rises 
over 1300° C, something weird starts to happen the rock around us starts to look like hot plastic. 
Here, the temperature is higher than the melting point of the rock but the pressure is so high that 
it’s still keeping the rock solid. It’s gooey. We’re now starting to push through even hotter material. 
It’s slowly inching from the bottom of the mantle toward the top in these giant convection currents 
over millions of years. And you might be wondering, why is the earth hot? Well it’s partly radiation 
and partly leftover heat from when the Earth formed by meteorites smashing against each other 
over and over again. The Earth is cooling very very slowly but we don’t need to worry about that, it’s 
going to take billions of years. In the meantime this stirring of hot goop brings enormous amounts 
of heat from the center toward the crust. But hang on… how do we know that? If humans have only ever 
been 12 km down how do we know that? “Yeah that’s a great question.” That’s Dr Megan Newcombe, a geologist 
and vulcanologist at the University of Maryland. She told me that studying the deep Earth is like 
being a detective. “We can’t get there so we have to put together all of these indirect pieces of 
evidence to work out what’s going on down there.” Ancient scientists had a lot of theories about 
what was going on down there. Some thought the Earth contained a central fire with underground 
lakes and lava chambers. Some thought it was hollow, maybe a set of concentric shells with life on each 
ring. This is the basis for the “Hollow Earth” in the Godzilla movies. Isaac Newton suggested that based 
on observable gravity, the stuff in the center must be denser than the stuff at the top. But it wasn’t 
until the early 20th century the scientists could prove that the Earth had a central core and there 
were several different layers above it. These breakthrough discoveries were thanks in large part 
to earthquakes. During an earthquake, we feel it up here, but it’s also sending seismic waves down. 
“They are just like sound waves but there are two main varieties: There’s P waves and S waves.” P waves 
can travel through liquids and solids but S waves can only travel through solids. They also behave 
differently depending on the density of the rock that they’re moving through. Which means that by 
measuring what waves end up at different detectors all over the surface, scientists can understand 
what’s going on between them. That’s one big way that we’ve learned about how the mantle differs 
from the crust and it also taught scientists something else: “We find this S wave shadow on the opposite 
side of the planet that tells us that for some reason the s-waves couldn’t propagate all the 
way through the Earth and that tells us that there’s a liquid layer down there.” That liquid 
layer is part of what we now call the outer core. We’ve now crossed into a liquid soup 
of metals cooking at around 4,400° C. Here temperature has won the fight against pressure 
and we should all be extremely grateful for the hot metal soup down there because its constant 
churning generates enormous electric currents which in turn create the magnetic field around the 
Earth. And without that, cosmic radiation would just end all life as we know it on the surface. But 
this gets weirder: Our magnetic field sometimes actually just reverses. The North and South 
poles slowly swap places. And we know that because… “you can actually read off all of those 
magnetic reversals on the ocean floor.” Yeah! “As basalt is erupted along our mid ocean ridges, it 
freezes in a record of what the magnetic field was at that time and as the plates continue to 
get created and pulled apart, we can read them off like a barcode.” So what you’re telling me is 
there’s a barcode for our magnetic fields at the bottom of the ocean and scientists have learned 
to read it? “Mhm!” Excellent. You can read this too: it flipped here, it flipped here, it flipped here. 
The last reversal was around 780,000 years ago. When will it flip next? We’re not sure! But I 
had a much more basic question about all of this: Why is the earth in layers? “You’re going to 
like this one: At the beginning we were a magma ocean. The entire Earth was molten liquid. During 
that time the densest materials which are the iron metal sank and formed the core and then 
the lightest minerals floated to the top and the densest minerals settled to the bottom and so 
we’ve segregated by density.” Okay we’ve almost made it through all of those layers we are deep 
inside the Earth’s outer core and then you hit something. Hang on though if you’re going on a 
real life adventure, you might want to take this. surface of the Sun and for a long time we didn’t 
know it was there. We assumed that everything in the core was molten because why would it be solid? 
But then these two scientists were studying more of those seismic waves and they noticed that when 
those waves got to the center they started to act weird! Waves were being bent and reflected by 
something in the middle. The math only made sense if there was a solid core inside the larger 
liquid one. Other scientists confirmed this theory with more precise recordings of the waves better 
computational models. So why is it solid? Well it turns out that the pressure here is so insanely 
high that the iron atoms literally can’t move and what’s even stranger is that more recently 
scientists have found slight discrepancies in the data that seem to show Earth’s core is rotating 
at a different rate than we are at the surface and that rate changes. It is speeding up and slowing 
down separately from what we’re doing up here because why not! Very precise seismic data and 
computer models of our magnetic fields show that the Earth’s inner core seems to be speeding 
up and slowing down on a roughly 70 years cycle you might have seen headlines that the earth’s 
core is reversing but that’s not true. It’s just that if you were on the outside of the inner 
core and I’m on the surface it would look to me like you were going the opposite direction if 
you’re slowing down even if we’re both moving the same direction. But this iron core is no match 
for our magical digger. We are going straight through it until finally we did it. We made it 
to the very center. We are now 6,400 km from the surface the pressure around us is 3.6 million 
times what it is on the surface. But in lots of ways what’s around us right now is still a big 
mystery. Like is there an inner inner core? Some scientists think that there might be there’s 
new seismic data that suggests that the iron atoms are packed differently way on the inside 
but others aren’t so sure that that’s enough to call it a new layer we’ve been wrong before but 
we’re becoming better detectives. Can we predict the Earth’s magnetic field and when it might 
flip? Scientists are building experiments that mimic the inner earth to learn more. Now the 
cutting edge is finding new ways to analyze seismic waves with better computational models 
and even finding ways to replicate the deep Earth conditions here on the surface. We are a 
curious species locked on the outside of our own home and we’re doing everything we can to peek 
inside. That’s what’s so cool about science and