This episode is sponsored by Brilliant The deepest hole mankind has ever dug was 12 kilometers deep, we’ll have to do several
hundred times better to get to Earth’s center, but if we can do it, we’ll gain access to
resources beyond our imagining. So today we’ll be looking at how to go about
accessing the Earth’s mantle and core—and the reasons we might do that. We should start by acknowledging that Earth
is immense. So often on this channel we look at scales
of celestial bodies that dwarf our pale blue dot, which makes it easy to forget that that
dot is still huge. If you could somehow explore 2000 new square
miles of the Earth’s surface every day, it would take you 80 years just to see all
of the land. And those would be busy days, because there
are over a million mountains to climb on this planet, and over 100 million freshwater lakes
to swim in. But the Earth’s immense volume is even harder
to visualize than its surface area. Humans occupy only a tiny portion of that
volume, a few tens of meters above and below the wrinkly surface. Beneath us is the real Earth, a trillion times
more mass and space than we use on the surface. We often talk about mining other bodies in
space, but you would need to harvest every rocky object in our solar system—Mercury,
Venus, Mars, all the asteroids and minor planets, all the moons around all the gas giants—to
gather as much material as we already have right under our feet. Steel production has been over a billion tons
per year and rising for decades now, and it might hit 2 billion tons per year in our lifetimes. But there are 2 billion, trillion tons of
iron inside the Earth. At our current rate, it would take us a trillion
years to deplete our local iron supply, if we could reach it all. All of the rare elements we plan to mine from
asteroids are far more abundant on Earth too, it just appears that crossing hundreds of
millions of kilometers of radiation soaked vacuum to get those elements will be easier
than digging them up. The best estimates on gold is that there’s
enough here to cover the Earth’s entire land surface in shiny coins like Scrooge McDuck’s
vault. But 99% of Earth’s gold is not in the crust
or mantle, but the deep core. But of course, this is only a best-guess estimate
from available data, which is limited. I’m often asked whether dark matter is real,
given we can’t even see the stuff, which makes many folks dubious. I typically reply that we can’t see what’s
in the Earth’s core, either. In fact, we can actually “see” dark matter
better than we can our own core, based on its gravitational effects on galaxies. Figuring out Earth’s composition is way
trickier and does involve a lot of estimates that get refined and adjusted over time. We do get some data, mostly about the upper
mantle, from volcanoes and oceanic trenches, though even these peeks below the crust are
hardly a direct sample, so we have to infer a lot of things. We can, for instance, look at meteor samples
or the metallicity of our Sun to determine their chemical compositions. Since everything in our solar system condensed
into their current forms from the material of the same planetary nebula, we can then
assume they represent a solid sample of what our world would have been made from. We can also use seismic data from earthquakes
and volcanic eruptions to study the rate at which seismic waves travel around the world
and, based on things like speed, coherence and refraction, infer the makeup of Earth,
since seismic waves behave differently when passing through different materials. But these data are only informed models, and
as science has shown us again and again, you’re almost guaranteed to encounter surprises when
comparing a model to the physical reality. So instead, we need to get down there and
get ourselves a nice good look. All the way down to the heart of the earth. We only need to do it once to know that it’s
possible, and once we determine it’s possible, such a journey to the center of the Earth
will open countless new cool opportunities--like travel. Points on Earth that are on exact opposite
sides of the planet are called antipodes. Jump “down” a vacuum tunnel in Beijing
and the first leg of your journey would speed you up so much that you’d have more than
enough energy to fly back “up” the shaft and emerge in Buenos Aires, the almost exact
antipode to Beijing. The entire trip would take about 84 minutes
and would require no fuel. And if we can drill one tunnel through the
Earth, we can drill multiple ones. We could use magnetic levitation trains to
traverse curved shafts through the mantle and the core and instead of spending 20 hours
on a plane flying from New York to Vietnam, you’d get there in a tiny fraction of that
time, again spending no fuel. Drill enough of these tunnels, and suddenly
Earth seems to become much, much smaller. Of course the Earth’s core can provide a
lot of power too. Earth absorbs only one billionth of the Sun’s
total solar output, but even so, compared to those levels of energy, geothermal power
is quite weak. Nevertheless, geothermal power is still an
enormously powerful source of energy--it is estimated that 47 terawatts of heat energy
is radiated from the core to the surface. It might not be a bad idea to bleed off some
of that heat while at the same time putting it to work. An interesting notion; we’ll get to that
later. As you’d suspect, the main issues for drilling
through the Earth are the immense heat and pressure, both of which climb the deeper you
go. However, we have a method for dealing with
ultra-high pressure called active support, as discussed in the Space Towers episode. Interesting notion there that we’ll get
to later, but just because the planet’s quite hot as we get deeper down, doesn’t
mean it needs to be or that it behooves us to let it be if we can change that. It is very hot and very high pressure and
those are big issues for drilling deep. However we have a technology for dealing with
ultra-high pressure that’s quite suitable for making tunnels from and which in general
works better when cooler, that active support technique, and if we’re willing to go all-in,
we could tunnel out the Earth with just modern technology by first cooling the planet. Of course doing such a thing would fundamentally
alter our planet, not just a bit of landscaping either but turning it into an artificial thing. However, that’s nothing really new to us,
when it comes to getting more living space you can go find more caves to dwell in or
you can cut them out deeper and use the rock you removed to expand our habitable space. You can get a lot more living room building
out of stone than cutting into it too. Our houses are mostly empty air after all. Cut a cave out to have twice as much living
volume and you’ve got enough stone to build a hundred times as much living area. And nicer living area at that, caves are nasty
places to dwell, and of course our ancestors rarely lived in them, mostly using them as
temporary shelters while on the move. But as a basic concept, that’s pretty much
humanity in a nutshell, we like nature but we’re clever and like to use our brains
to improve it. A planet is just a random pile of cosmic garbage
crunched together by its collective gravity, and our whole ecosphere is a thin layer of
scum growing on it. We can do much better. Indeed we’ve looked at alternatives to living
on planets often on the show, see the Megastructures series, particularly O’Neill Cylinders and
Dyson Spheres, but today we’ll look at improving our planet. To do that we have to start somewhere though
and step 1 is just finally making it all the way through the crust to the mantle, which
would get us about 1% of the way to the core, and sadly the easiest 1%. Our first two big attempts, Project Mohole
and the Kola Superdeep Borehole didn’t get us there though the latter drilled over 12
kilometers down. One of the newer plans revolves around the
Japanese deep sea drilling vessel, Shikyu, which hopes to drill through a thinner piece
of crust, hopefully getting started by 2030. Now that’s a purely scientific endeavor,
not industrial, but it’s where you start. It will let us learn more about the actual
composition of the mantle and the boundary layer between it and the crust, the Moho Discontinuity. This may also give us far better insights
into how Earthquakes occur and offer us a means of early detection. Seismic waves move quite fast, many times
the speed of sound in the air, but even a minute of advance warning could save many
lives. However, if you know how something works,
and you can get your hands on it, you can potentially take action too, don’t rule
out being able to model it so well we could detect earthquakes well in advance, and potentially
even use controlled detonations to release the pressure more safely, which we may look
at more in the future. But that’s small stuff, when it comes to
getting down there for raw materials and energy, we need to go big, and in some ways that’s
easier. It’s very hard to bore a deep skinny shaft,
slow work and constant issues drilling and shoring. A wider shaft is more work but in some ways
less hassle. There are many ways you might do this and
one we thought up involved detonating a series of atomic bombs and so we nicknamed it the
Nuclear Jackhammer. As you might guess it uses those bombs to
excavate material for the hole. Thing is, when you’re cutting down into
rock, be it from the bottom of the ocean or from land, you’ve got rock or magma at the
bottom of the shaft under much pressure and air or water above, providing less pressure. Now that’s fine if you want the magma to
constantly bubble up the shaft to be extracted for minerals, and indeed that’s the concept
behind the Moho Straw we looked at in Colonizing the Oceans last year for mineral extraction,
but it does make it a pain to drill deeper and shore up those tunnels. That Moho Straw by itself is quite handy,
you can build immense thick towers in the ocean from floor to sky, and indeed deep down
to the magma and all the way up to space, using active support to hold the tower up,
powered by heat engines using that magma for the heat and water for cooling and working
fluid, and just gorging yourself on all that energy and raw materials. But again, not good for going way deep, as
that magma is going to be constantly flowing up to fill the hole you just cut, or bombed. Hypothetically you could run this whole thing
with a drilling fluid at the bottom that was heavier than rock, something like mercury
or molten lead. Then your newly blasted rock and magma will
float up to the top of that for scooping off while the weight of that dense metal keeps
the pressure on the bottom of the hole to prevent a flood of magma. This needs to be big, because you’re using
nukes to do the cutting, and this also involves a lot of the heavy fluid, potentially billions
of tons of it, and losing some as it seeps into the magma and gets blown clear by the
atomic charges. Though this isn’t as big a deal as one might
think since the whole point of such an endeavor is to gain access to huge quantities of metals. However, it might makes more sense to steal
a page from our spaceship propulsion options, namely the Orion nuclear pusher plate design. That’s a spaceship that runs by dumping
atomic bombs out the back and detonating them at a short distance, where they slam into
a big thick metal plate on a spring which absorbs the shock and translates that into
a smooth acceleration for the rest of the ship in front of it. We’re going to do the same thing here, using
a massive metal plate with a shaft we can pop open to drop a nuke down. When the nuke goes off it shoves the plate
up while blowing more matter clear from the shaft. Though you might use more of Archimedes-Screw
drill bit arrangement that was being made to spin by the blast too and lifted that last
round of matter clear, or some similar arrangement. That bit or screw or plug, our jackhammer
tip, is very heavy and is pushing against the magma below. After each blast it’s going to drop back
down, shoving the newly blasted material over it. This thing is nothing but millions of tons
sturdy heat resistant metal full of cooling and radiating equipment that is probably suspended
on the end of super-tensile materials like graphene or carbon nanotubes to help keep
it lined up and dropping back down correctly, or a huge spring shaft with another heavier
plug above it adding more weight. You can also be generating insane amounts
of power off this too, as it’s as much an engine as a jackhammer. You’ll be dropping your shoring material
down as you go, on the side of this tunnel, and we’ll get to explaining what that’s
made of in a moment, but obviously you don’t want your nukes going off right next to your
pusher plate but deep down enough that the rock is absorbing most of the blast. So your nuke is likely to be along the lines
of a bunker buster bomb. You slam it down real fast and hard into the
next layer you’re blowing, possibly by having a big mass driver in that jackhammer shaft. This makes our jackhammer a mix of atomic
bomb and kinetic bombardment device, akin to the ‘Rods from God’ concept, a very
sturdy very fast dense-tipped projectile with a nuke in it. You might use raw slugs of uranium accelerated
down that shaft so fast they crunch and detonate without need of a chemical explosive to initiate
the process. You might shoot one down a bit below critical
speed to detonate that cuts down a ways and follow that immediately with another moving
a bit faster and setting off the blast when it hits, indeed this is essentially the gun-style
nuke used in more primitive atomic bombs. Possibly overkill as you might be able to
come up with alloys that can handle the heat and pressure and just drill conventionally. Such materials would make shoring up the tunnels
easier too, but we have a technology called ‘active support’ that we often discuss
using for making things like launch loops and orbital rings that can handle the pressure. You can see those episodes for detailed discussion
of the basic technology, but as a quick recap, classic materials can only handle so much
pressure before they crunch under the force, but by making a tube with matter spinning
around it very quickly, kept on course and sped up by magnets inside the tube, we can
create something absurdly strong. As a basic concept, think of a hose with no
water going through it, you can step on it and crunch it easily enough, but if we turn
the water on it presses back, it gets harder, wrap that in a loop with a pump on it propelling
the water around and you’ve got something fairly sturdy. Active Support of this type simply dials that
up to 11, allowing you to make something that can withstand immense pressure, so long as
you can provide power to it. Of course you don’t have to supply power
if you have superconductors but those don’t work at room temperature, let alone a thousand
degrees, so we either need way better superconductors or way better cooling or lots of power, or
all of the above if we can. Potentially you might be able to rig up that
matter stream inside the support ring to be a coolant too, but more to the point there’s
an awful lot of power available when you’re cutting deep into the Earth. Indeed our job gets easier if we remove that
heat and turn it into power. We can tunnel and mine and build deep down
way easier if it’s not hot. If we can tunnel down there and keep those
tunnels shored up, that also provides an amazing transport system too, one that follows shorter
routes than curving around the planet and so long as you keep them as a vacuum, lets
you run vacuum trains akin to the Hyperloop concept for essentially no power. You literally just ‘fall’ to your destination,
a gravity train. You can also use such tunnels for more than
just surface travel, a great big long tunnel can also be used as an acceleration tube to
get to space, see the Mass Drivers episode, though of course this requires you add a lot
of energy, but all that heat down there makes a great power source to run such a thing too. Now if you’re wondering why the center of
the planet is hotter than the surface, the bit that gets hit by the Sun’s warming light,
there’s two major reasons. First, it takes a ton of energy to lift matter
out of a gravity well, think about all the heat and energy of rockets lifting small cargos
into space, and you had all that heat and energy added when the matter fell down originally
to clump up into a planet. Left to itself it will cool down but this
is a process dependent on size, small stuff cools far faster and has less heat per unit
of mass too, due to its lower potential energy density from its weaker gravity field. We’re talking billion-year time frames for
large planets. To add to that, Earth’s core has a ton of
uranium in it producing heat as it decays, adding about 50 trillion watts of new heat,
quite a good deal more than our electric production nowadays though fairly small compared to what
we get from the Sun, which is around 4000 times as much. But it’s all down in the core which is quite
the insulator, so that slows the cooling of Earth immensely. The total heat in the center of the Earth
we’d need to remove to get it down to comfortable temperatures is over a million, trillion,
trillion joules. Needless to say just yanking all that out
would cause all sorts of problems as the density of materials is often highly variable with
their temperature and you might get mega-Earthquakes doing that too fast. This isn’t much of an issue though because
even if we tried to cool the planet down over a few thousand years by just running radiators
down deep and into the oceans to move that heat to the surface to radiate away, it would
involve emitting around a hundred times the heat Earth currently does. That would be one enormous power plant to
say the least, but not a very helpful one since we’d be hotter than Venus while that
was going on and presumably thoroughly dead, though we’ll discuss habitats inside such
heat momentarily. So you either have to go quite slow cooling
down the mantle and core, millions of years, or you have to erect truly massive space towers
for conducting the heat away to huge space-based radiating panels. We did discuss those devices, what we called
FORESTs, Fractal Obelisk Radiation Emitting Space Towers, in our Matrioshka Shell Worlds
episode. They’re particularly handy in this application
too since you can be extracting all that mass from the mantle to build many layers of concentric
spheres around Earth – using active support, running on all that power – to create huge
new living surfaces for humanity and the rest of Earth Life. Of course you may have another way to purge
that heat. We’re not really sure how the thermodynamics
of black holes works, but it’s been suggested you could use them as heat dumps, and black
holes are handy for keeping your gravity at the level you want if you’re hollowing out
planets or expanding their surface area, see Colonizing Black Holes for details. Normally black holes are millions of times
more massive than Earth or more, but that’s simply because of the forces needed to generate
an implosion sufficient to cause one naturally. Supernovae in very big stars occur when the
inner core has fused all the way up into iron and a detonation occurs in the layer above,
blowing the higher layers away and blowing that iron core into a tiny black dot. We’ve discussed artificially making smaller
ones before and one of the methods is to take a giant iron ball and implode it with a lot
of nukes. It’s conceivable you might be able to do
that to our own core, though doing it in a fashion that lets you wrap an active support
shell around it right after and without wrecking the planet is a lot trickier, but theoretically
possible. Once you have, you can adjust your gravity
to whatever you like, feeding it cheap matter like hydrogen, and this lets you circumvent
the problem with gravity changing as you hollow out a planet or expand it. You might be able to move lots of heat energy
down into that black hole at that point, though you can also use such things as enormous hyper-efficient
power generators too. Again see the Colonizing Black Holes and the
Matrioshka Worlds episodes for details. Lastly, we could just go live down there in
the heat. Better alloys are always nice but we could
build big spheres down in the magma with tubes rising to the surface, like a big thermometer
with that tube and ball on the bottom. You make that shell many layered, with the
outermost one being all about handling pressure and temperature and the next being full of
water pumping through to remove that heat, or some other coolant fluid, rising back up
to the surface and presumably running a lot of power generation along the way. You may want additional layers for redundancy
or added protections but your last layer is going to be two sheets with nothing in between,
a vacuum, same as a vacuum flask for keeping coffee hot or liquids cold in a thermos. This would be where folks lived, and you might
have ports on the bottom or side to let magma in that you could run mineral extraction and
manufacturing on, or just ship to the surface for that. Not that folks have to live there but they
could, and that shaft down need not be rigid. Ideally you wouldn’t want it to be so that
you could sway around in the various seismic waves. Indeed it could be like a pendulum, swinging
around slowly down there and attached at the top at one or two points. If you have two it has to swing in basically
a straight line, those could also serve as surface-to-surface gravity train paths. The top in this case probably being the bottom
of oceans, but you could always have transport lines back to the surface. Or just have a city down there, as we discussed
in Colonizing Oceans. For the places you’ve cooled down, or insulated
well and sturdily, we do have a lot of other options we already looked at in Subterranean
Civilizations. Such things take a lot of power to make particularly
livable, between lighting and cooling, normally but in this case that’s effectively free
since you’re using heat engines with the mantle or core as the hot reservoir, not burning
some fuel to make the heat, and you actually want to remove all that heat anyway. I should note before we wrap up that this
has applications off Earth too. Some worlds are far easier to access the core
from as they’re less massive and cooler to begin with, for others this might be the
only way to really live on them, for Chthonian worlds larger and hotter than Mercury or Venus
or for living in the depths of gas giants. You might also want to live this deep to protect
yourself from an enemy, since the surface of worlds is quite exposed and easily attacked,
and cities deep under a planetary crust are very well protected. It’s not necessarily a bad place to be putting
bunkers for keeping a protected seed of your civilization. Lastly, if something happened to Earth to
take away our sunlight, like we got ejected from the solar system by some rogue planet
or black hole passing through our area, these techniques can be used to save your civilization
as the surface goes dark and starts freezing. You could burrow ever deeper as things slowly
cool down, and as I mentioned, the time frame for a world like Earth with a molten core
to lose all its heat, even with sunlight gone, is many million of years. That’s lots of time to figure out alternatives
like fusion to provide new power. Or even drift through deep space and use some
of that energy and mass to nudge yourself to a trajectory in a habitable orbit around
another star. Somewhat of an amusing approach to going up
and out to new stars, digging down into our world’s depths first, but sometimes you
have to go backwards to go forwards, or downward to go upward. We’re in the holiday season so let me knock
off something from your holiday to-do lists. Gifts! If you’re watching this show, you probably
tend to feel like I do that knowledge is one of the best gifts you can give someone. If you know someone who likes to solve puzzles
or find out how things work, I’ve got a Brilliant gift suggestion for you… Brilliant. Brilliant is an online learning community
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their mobile apps offline feature lets you take courses even when you’re not getting
a good signal while traveling for the holiday season. This year get the gift of knowledge for your
loved ones by gifting them Brilliant, it’s such a fun way to nurture curiosity, build
confidence, and develop problem solving skills crucial to school, job interviews, or their
career. Go to Brilliant.org/IsaacArthur and grab a
gift subscription to help your loved ones finish their day a little smarter. Speaking of fun and gifts, next week we’ll
have some fun taking a look at Space Pirates, folks who tend to help themselves to other
people’s gifts, and we’ll see what form piracy might take in the distant and even
not so distant future in space. But before then we’ve got a special Bonus
Episode coming up this weekend, Paranoid Aliens, and we’ll ask what it might be like if instead
of us being paranoid about aliens, they were paranoid about us. And two weeks from now we’ll close out 2019
on SFIA by taking a look at Interstellar Civilizations and asking how Time affects them, from the
incredibly long signal delays and travel times, to just the notion of some civilization trying
to maintain stability when spread across a million stars and a million years. For alerts when those and other episodes come
out, make sure to subscribe to the channel, and if you’d like to get yourself a Christmas
gift, take a peek at some of our awesome SFIA merchandise on our website, IsaacArthur.net. Until next time, thanks for watching, have
a great week!
