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  • [♪ INTRO]

  • In 2003, the journal Nature published a paper

  • describing a rather unusual proposal.

  • The author suggested that scientists use a nuke to crack open

  • the Earth's crust and then toss in a vibrating,

  • grapefruit-sized recorder filled with scientific instruments.

  • The whole mess would sink through molten rock and metal

  • until it reached the Earth's core a couple of weeks later.

  • Now, this article was not actually a supervillain announcing

  • their dastardly plans.

  • It was more of a tongue-in-cheek thing.

  • The goal was to illustrate just how hard it is to study

  • the inside of the Earth.

  • I mean, we can see billions of light-years through space.

  • But when it comes to understanding what's beneath our feet?

  • That is actually much harder.

  • After all, space is see-through.

  • Rock is not.

  • Still, today we know a lot about how the Earth's interior is organized.

  • There are distinct layers, for instancethe crust,

  • the molten mantle, a liquid outer core, and a solid metallic

  • inner corewith even more transitions and subdivisions in between.

  • But figuring it all out has involved some inventive thinking

  • across multiple scientific disciplines, and it's taken scientists

  • to some surprising places.

  • So, here are seven ways we have peered inside our planet.

  • Let's start with the obvious one: just digging a big hole.

  • No one has been able to dig down to the mantle yet,

  • but that doesn't mean people haven't tried.

  • In 2005, for example, an international group of researchers

  • called the Integrated Ocean Drilling Program set out

  • to reach the crust-mantle boundary.

  • To do this, they targeted a thin point of crust located

  • on the floor of the North Atlantic ocean.

  • In the end, they weren't able to get there.

  • After drilling down for more than 1.4 kilometers,

  • they missed the thin patch by only about 300 meters.

  • Even if they haven't successfully made it through,

  • these types of projects have helped scientists learn more

  • about the crust and seafloor, such as the unique microbial

  • communities we can find down there.

  • And we haven't given up drilling yet.

  • A team of Japanese researchers, for instance, is looking at

  • trying their hand somewhere near Hawai'i in the future.

  • A simpler idea is to study rocks and geological activity

  • right here on the surface.

  • Studying volcanoes and fault lines, for example, can teach us

  • more about plate tectonics, where hotspots in the mantle might be,

  • and how magma reservoirs form underneath volcanoes.

  • There's a lot we can learn just from looking at rocks --

  • especially older ones.

  • Xenoliths are parts of Earth's mantle brought to the surface

  • trapped inside other bits of volcanic rock.

  • The most informative -- and spectacular -- type of xenolith

  • are diamonds.

  • Diamonds form only under very specific conditions at depths

  • of 150 kilometers or more in the upper mantle.

  • So anything trapped in the diamondor anything that comes up

  • alongside itmust've come from at least that far down.

  • Scientists can also examine the chemical makeup of ancient rocks.

  • That's revealed that, among other things, some of what spews out

  • from volcanoes isn't fresh mantle material, as you might expect --

  • but rather elements from old, recycled crust.

  • In 2016, researchers measured the ratio of magnesium isotopes

  • in solidified lava from the French island of Martinique.

  • Then they compared that to the ratios previously seen in

  • other crust and mantle material.

  • They found that the magnesium seen in the Martinique lava

  • looked like crust stuff.

  • The team thinks that certain elements might get squeezed out

  • of the rocks along with water to travel towards the surface

  • as bits of old crust sink.

  • In other words, bits of surface stuff seem to sink deeper

  • into the Earth, then get brought back up.

  • Understanding how these fluids travel could help us

  • better understand how volcanoes and earthquakes work.

  • Digging, looking at rocks, and studying volcanoes is

  • all pretty hands-on stuff.

  • But there are also ways to look inside the Earth without

  • heading into the field.

  • Such as using seismic waves.

  • These are the vibrations created not by us, but by earthquakes

  • that spread through the Earth like ripples across a pond.

  • Different densities of rock bend or reflect those waves.

  • By analyzing the pattern, researchers can make inferences

  • about the shape of things underground.

  • This approach was key to one of our first big breakthroughs

  • in understanding Earth's interior.

  • In 1929, Danish seismologist Inge Lehmann was examining seismic waves.

  • At the time, scientists knew that Earth had some solid

  • and some liquid layers -- but they thought the core was molten.

  • If that was true, the waves from an earthquake should

  • spread out smoothly from its epicenter.

  • But Lehmann noticed that some of the vibrations seemed to

  • bounceback towards the surface.

  • The only explanation, she figured, would be that they were

  • reflecting off something big and rigid at the center of the Earth.

  • We now know that thing is our planet's solid inner core.

  • Today, we're still using seismic waves to learn more

  • about Earth's interior.

  • In 2019, for example, scientists found a kind of ironsnow

  • falling from the outer core towards the inner one using

  • seismic wave data.

  • We can also learn a lot from noticing when things get weird.

  • Irregularities in the planet's properties happen for a reason,

  • and sometimes that points towards a cause that we can't actually see.

  • For instance, there is a line known as the Brunswick Magnetic Anomaly

  • that runs through Alabama and Georgia where Earth's magnetic field

  • seems unusually weak.

  • Scientists can map it thanks to magnetometers,

  • which measure the strength of a magnetic field.

  • Magnetic anomalies can be caused by the composition of rock

  • in an area.

  • A streak of magnetite ore, which contains iron,

  • may have an unusually strong magnetic field.

  • On the flipside, a particularly weak field might mean

  • there's a significant lack of magnetic material.

  • Sedimentary rocks like sandstone often contain

  • relatively little metal.

  • These anomalies can also teach us about geologic history.

  • For example, a 2014 study suggested that the Brunswick anomaly

  • was due to rock left behind millions of years ago as

  • the supercontinent Pangea split up, separating

  • North America from Africa.

  • We can also look for anomalies in how well the crust

  • or upper mantle conduct electricity.

  • Earth's magnetic field varies naturally over time,

  • and changing magnetic fields create currents of electricity.

  • Scientists can measure that current by planting electrodes

  • in the ground.

  • Then, by comparing how changes in the magnetic field lead

  • to changes in current, they can calculate how well the rock

  • below conducts electricity.

  • Depending on how things are set up, this technique can peer

  • hundreds of kilometers below the surface, revealing properties

  • like the temperature and even composition of the material down there.

  • It can also help researchers calculate how much water is trapped

  • in the rock.

  • In fact, one study found that there might be as much water

  • in the mantle as in all the oceans, locked up in water-containing

  • minerals like ringwoodite.

  • So far, we've looked at real measurements in nature.

  • But figuring out what those mean often relies on

  • Scientists' models and lab experiments.

  • The pressures and temperatures deep within Earth can be extreme,

  • resulting in physics and chemistry that behave differently

  • than up here on the surface.

  • One tool scientists use is the diamond anvil cell, which consists

  • of two small, flawless diamonds ground to precision points

  • and mounted on pistons.

  • Since pressure is force divided by areaaccording to the math

  • when the pistons apply their huge force to the tiny points

  • of the diamonds, the pressure can be ridiculous.

  • In 2009, for instance, scientists reported subjecting an iron alloy

  • to two hundred billion Pascals of pressuremore than half

  • of what it would be inside the inner core!

  • Amazingly, to find some of the planet's interior anomalies,

  • we actually have to go to space.

  • This is especially true for one particular kind --

  • gravitational anomalies.

  • From 2002 until 2017, NASA's GRACE mission used two spacecraft

  • more than 500 kilometers above the Earth's surface

  • to map out fluctuations in Earth's gravitational field.

  • The result was maps like these that show where gravity

  • is oddly strong or weak.

  • If you think back to high school physics, you might remember

  • that the more mass something has, the more gravity it exerts.

  • That means fluctuations in Earth's gravitational field can point

  • to parts of the planet that are more or less dense than others.

  • And since the crust, mantle, and core are made of stuff with

  • different densities, scientists can translate these variations

  • into physical understanding.

  • For example, if the crust in an area is known to be less dense

  • than the material in the mantle, weaker gravity might point