The Mystery at Earth's Center

People have climbed to the top of the tallest mountains. People have gone to the deepest part of the ocean. People have even been to space! But there’s one place people have never been able to reach — the center of Earth.

Why Not?

Earth has layers, kind of like an onion. We live on the crust. It’s the thinnest layer, but it still goes 25 miles (40 km) deep. Underneath the crust is the mantle. The mantle is the thickest layer. It makes up most of Earth’s interior and is about 1800 miles (2900 km) thick. The outer core is below the mantle. Then there’s one more core below that, called the inner core. Put it all together, and the center of Earth lies about 3,959 miles (6,378 km) below your feet!

The deepest hole ever drilled went about 7.6 miles (12.2 km) deep. That’s only a third of the way through the crust and nowhere near the core. In 2019 a research ship tried to drill through a thinner part of the ocean floor. The drill only made it about 2 miles (3.2 km) down before the tunnel collapsed. One scientist has suggested using a lot of explosives to crack open a spot in the crust, but so far nobody’s tried that.

Not only is Earth’s core far, far down, but as the layers go deeper, they also grow hotter. The temperature at the bottom of the crust is about 1832°F (1000°C). It’s a toasty 6332°F (3500°C) at the bottom of the mantle. In fact, it’s so hot there that the rock stays melted into liquid. The core is even hotter. It’s almost as hot as the sun’s surface! So, even if we had a drill that could reach that deep, we’d need a way to keep it from melting.

The good news is that even though we can’t reach Earth’s center, we still have a pretty good idea of what’s there.

But If We Can’t Reach It, How Do We Know?

We may not be able to make it to Earth’s center, but there’s something that can… earthquakes! Earth’s crust is made up of gigantic plates that move very slowly. Sometimes the plates bump into each other. Other times they get stuck and then snap apart. The plates’ bumping or snapping releases energy that spreads as waves. These waves can move through the entire planet like ripples across a pond.

We feel some of the waves as an earthquake’s ground-shaking vibrations. Others travel deep into Earth. Scientists measure how fast the waves go and what paths they take. Then they use that information to build a picture of Earth’s insides. Think of it like a doctor using an X-ray to look inside a patient, but instead of X-rays, scientists use earthquake waves.

So What’s Down There?

We know Earth has a solid inner core. Scientists first thought the whole core was liquid. Then in 1929, a scientist named Inge Lehmann discovered that earthquake waves were bouncing off the inner core. She knew that if the core had been liquid, they would have moved through it. It had to be solid.

Lots of measurements and new computer models have told us more. The inner core is almost as big as the moon, and it’s mostly made of iron. The outer core is also made of iron, but it’s liquid. Like Earth itself, the cores spin. But they don’t move in the same direction. The inner core turns to the east. The outer core turns to the west. Scientists think the cores’ opposite turning is what creates Earth’s magnetic field, which protects the planet from the sun’s harmful energy.

Now for the Mystery

New research hints there might be something else at the deepest part of Earth’s center. In 2020, a team of scientists announced that they’d looked at how 100,000 earthquake waves moved through the inner core. Their study shows that how the waves move through the very inner part doesn’t match the rest. That could mean there’s something different there. It might have something to do with the way the center is changing as the core around it cools and expands. But nobody knows for sure.

Earth’s center remains a mystery.