# What is the Earth's core?

An episode of [Science for Kids / Science's Not Boring: The Kids Science Podcast](https://invisiblenetwork.ai/podcast/1896928296), 15 Aug 2026, 14 min.

Audio: https://www.buzzsprout.com/2624612/episodes/19617694-what-is-the-earth-s-core.mp3

Have you ever wondered what would happen if you dug a hole straight down through your backyard? You wouldn't just find dirt! Join Mira and Finn on an epic journey deep underground to discover the amazing, blistering secrets hiding right beneath our feet! First, we’ll explore the bubbling, oozing hot rock called magma, and find out exactly what happens when it blasts out of a volcano and turns into lava. Did you know the deepest hole humans ever dug had to stop because the rock turned into hot, sticky putty? It's true! But the wildest part is right at the very center of the planet. Discover a giant, swirling ocean of liquid metal and a solid iron ball that is literally as hot as the surface of the sun! Plus, find out how a brilliant scientist named Inge Lehmann used cardboard oatmeal boxes and earthquake waves to prove it was all there. You'll even learn how this giant iron center acts like an invisible superhero force field to keep everyone on Earth safe!

## Transcript

[0:00] SPEAKER_01: Welcome to Science Is Not Boring by kidopoly.com.
[0:03] SPEAKER_01: I'm Mira.
[0:04] SPEAKER_01: And I'm Finn.
[0:06] SPEAKER_01: Before we start, we have a huge shout out today.
[0:09] SPEAKER_01: A big hello to Lincoln, who is almost eight, and listens with his dad Caleb.
[0:14] SPEAKER_01: Hi Lincoln.
[0:15] SPEAKER_01: Hi Caleb.
[0:17] SPEAKER_01: Lincoln actually asked us all about the Earth's core, especially magma and lava, so this entire episode is just for you, Lincoln.
[0:24] SPEAKER_01: Great pick.
[0:26] SPEAKER_00: Yeah, I've always wanted to know what is actually inside the Earth.
[0:29] SPEAKER_00: I used to think if I dug a hole in my backyard with a plastic shovel, I would just find dirt all the way down to the other side of the planet.
[0:37] SPEAKER_01: Not even close, Finn.
[0:39] SPEAKER_01: What if I told you that right at the centre of the Earth, about 4,000 miles beneath our feet, sits a giant solid ball of iron, and it is literally as hot as the surface of the sun.
[0:51] SPEAKER_00: As hot as the sun?
[0:52] SPEAKER_00: But we're not burning up.
[0:54] SPEAKER_00: We're just sitting here in a studio.
[0:56] SPEAKER_01: Haha, exactly! Today we are taking a journey straight down to the centre of the planet.
[1:02] SPEAKER_01: We'll explore bubbling magma, the deepest hole humans ever dug, and an invisible shield that keeps every living thing on Earth safe from outer space.
[1:11] SPEAKER_00: Grab your heat-proof suits, we're going digging.
[1:14] SPEAKER_01: Okay, imagine the Earth is like a giant peach.
[1:18] SPEAKER_01: The part we stand on, the crust, is just the thin, fuzzy skin on the very outside.
[1:24] SPEAKER_01: That includes the mountains, the oceans, the ground under our houses.
[1:28] SPEAKER_00: So all the dirt and rocks I've ever seen.
[1:32] SPEAKER_00: That's all just the skin.
[1:34] SPEAKER_00: Yep.
[1:35] SPEAKER_01: The crust is only about 40 miles thick at its deepest points.
[1:39] SPEAKER_01: Underneath that is the next layer, the mantle, which is where things start getting really, really hot.
[1:46] SPEAKER_00: Let's just dig a hole to the mantle.
[1:48] SPEAKER_00: We have giant drills and massive machines, right?
[1:52] SPEAKER_01: People actually tried that.
[1:54] SPEAKER_01: In 1970, scientists in Russia started digging a massive hole called the Kola Super Deep Borehole.
[2:01] SPEAKER_01: They wanted to see exactly how far down they could go before the planet stopped them.
[2:07] SPEAKER_00: Well, how far did they get?
[2:09] SPEAKER_00: Did they reach the centre?
[2:10] SPEAKER_01: Nope.
[2:11] SPEAKER_01: They drilled for over 20 years and they only reached 12,262 metres deep.
[2:19] SPEAKER_01: That's about 40,230 feet, or 7.6 miles straight down into the crust.
[2:27] SPEAKER_00: 20 years just to go seven miles?
[2:30] SPEAKER_00: Why did they stop?
[2:32] SPEAKER_00: Did their drill break?
[2:33] SPEAKER_00: It was because it got way too hot.
[2:36] SPEAKER_01: Down at 12,000 metres, the temperature reached 180 degrees Celsius, or 356 degrees Fahrenheit.
[2:45] SPEAKER_01: That was almost twice as hot as they predicted.
[2:51] SPEAKER_01: Exactly.
[2:52] SPEAKER_01: The rocks down there were so incredibly hot that they stopped acting like hard, solid stone.
[2:58] SPEAKER_01: They turned into a sticky, hot plastic putty.
[3:02] SPEAKER_00: The drills just got stuck and couldn't spin anymore.
[3:06] SPEAKER_00: Ew, gooey hot rock putty.
[3:08] SPEAKER_00: That sounds like a total nightmare to clean up.
[3:12] SPEAKER_01: So the deepest hole humans have ever made barely even scratched the Earth's skin.
[3:18] SPEAKER_01: To get to the centre, we'd have to dig over 500 times deeper than that.
[3:23] SPEAKER_00: Okay, so if we can't drill past the crust because of the hot rock putty, what actually happens in that next layer, the mantle?
[3:31] SPEAKER_01: The mantle is absolutely huge.
[3:34] SPEAKER_01: It goes down about 1,800 miles or 2,900 kilometers.
[3:40] SPEAKER_01: It's mostly solid rock, but it is so blisteringly hot.
[3:44] SPEAKER_01: Anywhere from 1,000 to over 3,700 degrees Celsius, that over millions of years the rock actually flows and oozes.
[3:55] SPEAKER_01: Kind of like extremely hot, thick caramel.
[3:58] SPEAKER_01: Is that flowing caramel rock where lava comes from?
[4:02] SPEAKER_01: Almost.
[4:03] SPEAKER_01: When pockets of rock in the mantle get hot enough to completely melt into a liquid, it's called magma.
[4:10] SPEAKER_01: Magma is what we call melted rock when it's still trapped deep underground.
[4:15] SPEAKER_00: Oh I get it.
[4:16] SPEAKER_00: So what is lava then?
[4:18] SPEAKER_00: Are they the same thing?
[4:20] SPEAKER_01: Lava is just magma that has escaped.
[4:22] SPEAKER_01: When pressure builds up underground, that molten rock bursts out through the crust in a volcano.
[4:29] SPEAKER_01: The second it hits the surface and touches the air, it gets a new name.
[4:33] SPEAKER_01: Lava.
[4:35] SPEAKER_00: Whoa! How much lava can a volcano actually blast out?
[4:39] SPEAKER_01: A massive amount.
[4:40] SPEAKER_01: For example, during one famous eruption on the Reykines Peninsula in Iceland, the volcano was spewing out enough lava to fill an entire Olympic swimming pool every 20 seconds.
[4:54] SPEAKER_00: Every 20 seconds?
[4:56] SPEAKER_00: An Olympic pool is enormous.
[4:58] SPEAKER_00: That is so much hot rock.
[5:01] SPEAKER_01: And lava is insanely hot.
[5:04] SPEAKER_01: Usually between 700 and 1,200 degrees Celsius.
[5:09] SPEAKER_01: But as deep and hot as the mantle is, things get even crazier when we drop down into the next layer, the outer core.
[5:17] SPEAKER_00: I don't know if my heat-proof suit can handle this.
[5:20] SPEAKER_00: How hot is the outer core?
[5:22] SPEAKER_01: It can reach up to 5,700 degrees Celsius.
[5:26] SPEAKER_01: And unlike the mantle, the outer core isn't made of rock at all.
[5:31] SPEAKER_01: It is a giant swirling ocean of liquid metal, mostly iron and nickel.
[5:37] SPEAKER_00: An ocean of liquid metal?
[5:39] SPEAKER_00: That sounds like something straight out of a science fiction movie.
[5:42] SPEAKER_00: Does it have waves?
[5:44] SPEAKER_01: It doesn't just have waves, it's constantly churning and flowing as the earth spins.
[5:49] SPEAKER_01: This ocean of glowing liquid iron is about 1,400 miles thick.
[5:56] SPEAKER_00: Wait, if the outer core is a giant liquid ocean of metal, what's right at the very centre?
[6:01] SPEAKER_00: Is there an island in the middle of the metal ocean?
[6:04] SPEAKER_01: You're actually not far off.
[6:06] SPEAKER_01: At about 3,200 miles straight down, we hit the inner core.
[6:12] SPEAKER_01: This is a giant ball of mostly iron, about 760 miles in radius, which makes it roughly the size of Pluto.
[6:20] SPEAKER_01: And the temperature is around 6,000 degrees Celsius, or nearly 11,000 degrees Fahrenheit.
[6:28] SPEAKER_00: You said earlier that's as hot as the surface of the Sun.
[6:32] SPEAKER_00: How is the center of our planet a tiny Sun?
[6:35] SPEAKER_01: It's all leftover heat from when the Earth first formed, 4.5 billion years ago, plus heat from radioactive elements decaying.
[6:44] SPEAKER_01: But here is the truly mind-blowing part, Finn.
[6:48] SPEAKER_01: The inner core is hotter than the liquid outer core.
[6:52] SPEAKER_01: But it is completely solid.
[6:55] SPEAKER_00: Wait, what?
[6:56] SPEAKER_00: That doesn't make any sense.
[6:58] SPEAKER_00: How can it be solid if it's hotter?
[7:00] SPEAKER_00: Shouldn't the metal melt even more?
[7:02] SPEAKER_01: You would totally think so.
[7:04] SPEAKER_01: But because the inner core is at the very centre of the planet, the weight of the entire Earth, the crust, the oceans, the mantle, the outer core, is all pressing down on it.
[7:17] SPEAKER_01: The pressure is so intense that the iron atoms are squeezed completely tight.
[7:22] SPEAKER_01: They literally cannot move around to become a liquid.
[7:26] SPEAKER_00: That is wild.
[7:28] SPEAKER_00: But wait a minute, you said the deepest hole we ever dug was only seven miles down.
[7:33] SPEAKER_00: The centre is thousands of miles away.
[7:36] SPEAKER_00: How do we even know there's a solid iron ball inside the earth if we've never seen it?
[7:41] SPEAKER_01: That is one of the best stories in science.
[7:44] SPEAKER_01: In 1936, a brilliant Danish seismologist named Inga Lehman figured it out.
[7:50] SPEAKER_01: And she didn't use a drill, she used earthquakes.
[7:54] SPEAKER_00: Earthquakes?
[7:56] SPEAKER_00: How do earthquakes help you look thousands of miles underground?
[7:59] SPEAKER_01: When an earthquake happens, it sends invisible shock waves, called seismic waves, rippling right through the planet.
[8:08] SPEAKER_01: Inga Lehman studied how these waves moved.
[8:11] SPEAKER_01: She noticed that certain waves, called P waves, were bouncing and bending off something hard right in the middle of the earth.
[8:19] SPEAKER_00: So she basically used the earthquake like a giant planetary X-ray.
[8:23] SPEAKER_01: Exactly.
[8:25] SPEAKER_01: She organised all her earthquake data using index cards that she kept in cardboard oatmeal boxes in her garden.
[8:32] SPEAKER_01: And with just math, patience, and those earthquake readings, she proved the solid inner core existed.
[8:41] SPEAKER_01: She was a genius and she lived to be 104 years old.
[8:45] SPEAKER_00: She discovered the solid centre of the earth using oatmeal boxes and math?
[8:50] SPEAKER_00: That is the coolest thing ever.
[8:52] SPEAKER_00: I need to save my oatmeal boxes.
[8:55] SPEAKER_01: Science is all about solving mysteries with the clues you have, no matter where you find them.
[9:01] SPEAKER_01: And it's a really good thing we have the Earth's core, because without it, none of us would be alive today.
[9:07] SPEAKER_00: Really?
[9:08] SPEAKER_00: How does a hot iron ball keep us alive?
[9:10] SPEAKER_00: It's literally thousands of miles away, buried under solid rock.
[9:14] SPEAKER_01: Remember that ocean of liquid metal in the outer core?
[9:18] SPEAKER_01: As the earth spins, that liquid iron swirl, flows, and churns.
[9:23] SPEAKER_01: All that moving metal acts like a giant electrical generator.
[9:27] SPEAKER_01: It creates a massive, invisible magnetic field that stretches all the way out into space, completely surrounding the Earth.
[9:35] SPEAKER_01: Like a giant invisible force field.
[9:37] SPEAKER_01: Exactly like a force field.
[9:39] SPEAKER_01: The sun is constantly shooting out dangerous radiation and powerful solar winds.
[9:45] SPEAKER_01: If we didn't have our magnetic field blocking those winds, they would literally strip our atmosphere away and blow our air deep into space.
[9:54] SPEAKER_00: Oh no! We need our atmosphere to breathe and to keep the water from freezing.
[9:58] SPEAKER_01: We do.
[9:59] SPEAKER_01: Without the magnetic shield created by our swirling core, Earth would end up looking exactly like Mars.
[10:06] SPEAKER_01: A dry, freezing, rocky desert with no breathable air.
[10:10] SPEAKER_00: Phew! Thank you, swirling ocean of liquid metal.
[10:14] SPEAKER_00: I will never take you for granted again.
[10:16] SPEAKER_01: Here is one last fun fact for you, Lincoln.
[10:19] SPEAKER_01: The Earth's solid inner core is actually growing.
[10:22] SPEAKER_00: Wait, it's getting bigger?
[10:24] SPEAKER_00: How is it growing if it's trapped in the middle of the planet?
[10:27] SPEAKER_01: The Earth is very, very slowly cooling down over billions of years.
[10:32] SPEAKER_01: As the planet cools, the liquid metal at the very bottom of the outer core freezes into solid iron, attaching to the inner core.
[10:40] SPEAKER_01: It adds about one millimetre of solid iron to the inner core every single year.
[10:45] SPEAKER_00: That is so cool! The Earth is basically building its own giant iron snowball, one millimeter at a time.
[10:52] SPEAKER_00: Alright, Finn, time for the quiz.
[10:54] SPEAKER_00: Are you ready?
[10:55] SPEAKER_00: Okay, I'm ready.
[10:57] SPEAKER_00: I hope I remember all the numbers and temperatures.
[10:59] SPEAKER_01: First question.
[11:01] SPEAKER_01: What is the name of the seven mile deep hole scientists dug in Russia that got stuck in hot rock putty?
[11:11] SPEAKER_01: Next one.
[11:12] SPEAKER_01: What is melted rock called when it is still trapped deep underground before it erupts?
[11:22] SPEAKER_01: Question three.
[11:23] SPEAKER_01: What is the Earth's solid inner core as hot as?
[11:30] SPEAKER_01: Here's number four.
[11:32] SPEAKER_01: Which brilliant Danish scientist discovered the solid inner core in 1936 using earthquake waves?
[11:43] SPEAKER_01: Last question.
[11:44] SPEAKER_01: What invisible force field does the swirling liquid outer core create to protect our atmosphere?
[11:54] SPEAKER_01: Great job! Let's see how you did.
[11:57] SPEAKER_01: The first answer is the kola superdeep borehole.
[12:01] SPEAKER_01: They dug for 20 years and only reached 12,262 meters.
[12:07] SPEAKER_01: For question two, melted rock underground is called magma.
[12:12] SPEAKER_01: Once it bursts out of a volcano, it's called lava.
[12:15] SPEAKER_01: Number three, the answer is it is as hot as the surface of the sun, about 6,000 degrees Celsius.
[12:24] SPEAKER_01: Question 4.
[12:25] SPEAKER_01: It was Danish seismologist Inge Lehmann and her famous oatmeal boxes.
[12:31] SPEAKER_01: And the final answer is, it creates the Earth's magnetic field, which protects our air from solar winds.
[12:38] SPEAKER_00: I still can't get over the fact that there is a giant ball of iron right beneath our feet, as hot as the sun.
[12:45] SPEAKER_00: Every time I walk outside now, I'm going to think about it.
[12:50] SPEAKER_01: I know, right?
[12:51] SPEAKER_01: And the fact that Inga Lehman figured it all out using just earthquake shockwaves and cardboard boxes.
[12:58] SPEAKER_00: Science is truly wild.
[13:00] SPEAKER_00: If you thought Inga Lehman was a genius, or you love learning about magma and lava with us, please leave us a five-star review.
[13:08] SPEAKER_01: Yes, it's super easy.
[13:10] SPEAKER_01: Just scroll down on whatever app you're using to listen and tap those five stars.
[13:15] SPEAKER_01: It helps other curious kids find the show.
[13:18] SPEAKER_00: Oh, and don't forget to check out our site kidopoly.com.
[13:22] SPEAKER_00: We've got tons of fun learning games and activities waiting for you there.
[13:26] SPEAKER_01: And if you want to shout out on the show, like Lincoln and his dad Caleb, or just want to tell us your favourite science fact, email us at hello at kidopoly.com.
[13:37] SPEAKER_01: We can't wait to hear from you.
[13:39] SPEAKER_01: See you next time on Science is Not Boring.
[13:42] SPEAKER_01: Bye, everyone.

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