Hank green: Space may be the final frontier, but there's a lot here on Earth that we are never going to get a good look at, and I'm talking about stuff that's right underneath your feet. Well, maybe not right under your feet -- like, really really far beneath your feet. [intro music] Depending on whether you're in Denver or Florida, the center of the Earth is about 3,959 miles below your feet right now, and to put that number into perspective, the deepest that a person has ever been below the surface of the Earth is about 2.4 miles in a gold mine in South Africa. Truth is, we've barely cracked the surface of the crust of this planet, leaving thousands of miles of upper and lower mantle to go before you get to the hot molten core of nickle and iron that we all know makes up the center of our planet. Except that that's not entirely true. Yeah. There is indeed an enormous liquid furnace with a radius of thousands of miles raging beneath your feet right now (probably your butt, because you're probably sitting down; beneath your butt too), but that's just the outer core. There's also the inner core, where temperatures are as high as on the surface of the Sun. But unlike the outer core, the inner core is essentially a solid ball of nickle and iron. You've heard me right. This is popular misconception. The Earth is like an everlasting gobstopper with three different flavors, solid, liquid, and then solid again -- and it's not just, like, a teeny tiny little ball, either. The inner core has a radius of about 760 miles. That's 70% the size of the moon. So if the Earth is like this inexpensive basketball that we just picked up because no one in our office had one 'cause none of us go outside, then the core -- the solid inner core of the Earth -- would be about the size of this golf ball. Now, that doesn't look very big, but this is big frickin' Earth, and so that's a pretty good size ball of nickle and iron that, y'know, nickle and iron miners I'm sure would love to have access to, but they can't. But seriously, how is this moon-sized golf ball of solid iron and nickle possible surrounded by molten metal? Like, why isn't that melted? Well, to begin with, you thank an event with a name so awesome I couldn't even have made it up: The Iron Catastrophe. It happened about 500 million years into the Earth's lifespan. Back then, all the heat and radioactivity from all the materials smashing together that would later become the Earth grew so hot that it all became molten and gooey, and this is why the Earth is a sphere, which is convenient for us. When that happened -- when the Earth was a giant liquid ball -- all of that dense nickle and iron contracted because of gravity into the center of the planet. And there, it slowly cooled down, forming the solid inner core of the Earth that continues to expand today. But I know what you're thinking -- "It's, like, 9,800 degrees down there! Why isn't that big metal ball slowly melting away?" Because it's not only very very hot down there, it's also under an extreme amount of pressure, and as the pressure goes up, so does the melting point of iron, which is good, because we want the outer core to stick around for as long as possible. It's pretty important to our life here on Earth. And that's because it's the convection of all of those liquid nickle and iron metals that creates, through another really awesome phrase called "dynamo action", the magnetic sphere that surrounds our Earth. And that magnetic field has a rather vital job, which is to keep everything on Earth alive. When astronauts go up into space, they are no longer protected by the Earth's magnetic sphere, and thus they are exposed to huge amounts of radiation. This actually happens to you when you go up in airplanes -- the amount of radiation you are exposed to goes up dramatically. But here on the surface of the Earth, the magnetic field created by the Earth's core protects us from that radiation. But not all planets are so lucky. Scientists are pretty sure that Mars actually has a liquid core, but its dynamos shut down at some point; the convection is no longer going on, maybe because of some dramatic asteroid impact, and it no longer has magnetic fields, so if we're ever gonna live on the surface of Mars, we're gonna need a lot of radiation shielding. As for Earth, since we can't imagine a situation in which we could physically examine the core of the Earth, we'll never really know what's down there, All we can really say is that things down there are very different from what Jules Verne once imagined; though that would be pretty awesome too, it wouldn't be the dynamo that makes all life on Earth possible. Thank you for watching this SciShow Dose. If you wanna find out more about the Earth's liquid and solid core, we have links for you in the description. We're also curious what you want us to talk about, and if you have any questions, you can get to us on Facebook and on Twitter, and of course in the comments below. [endscreen]