How do we know what’s inside the Earth?

Nobody has ever been deeper than about 12 km, yet we know the core is iron and partly liquid. Here’s the detective work, from earthquake waves to space rocks to squeezing iron in a diamond vise.

The deepest hole ever drilled, Russia’s Kola Superdeep Borehole, reached about 12 km down. That’s roughly 0.2% of the way to the center. Nobody has ever seen the mantle up close, let alone the core.

So how can we say the center of the Earth is a ball of solid iron hotter than the surface of the Sun? It took more than a century of clever detective work, and the clues come from some surprising places.

Clue 1: Earth is too heavy to be all rock

In 1798 the English scientist Henry Cavendish used a delicate twisting balance and some lead balls to “weigh the world.” He found that Earth is, on average, about 5.5 times as dense as water, within 1% of today’s value of 5.51.

The rocks at the surface are only about half that dense. So something much heavier must be hiding deep inside. The best candidate is iron.

Clue 2: Earthquakes are planet-sized X-rays

When a big earthquake strikes, it sends waves through the whole planet, and seismometers all around the world record them. There are two main kinds:

P waves (“primary”) push and pull, squeezing the rock as they go, like a sound wave. They’re the fastest, and they can travel through solids and liquids.
S waves (“secondary”) shake the rock from side to side. They’re slower, and they can’t travel through liquids at all.

Both kinds bend and change speed as the rock around them changes. By timing exactly when each wave arrives at stations all over the world, scientists can work out what the waves passed through, a bit like a doctor reading a scan.

Fun fact: Today thousands of seismometers listen around the clock. Combining millions of wave arrivals lets scientists build 3D maps of the hot and cold blobs inside the mantle, a technique called seismic tomography.

The discoveries, one layer at a time

1906: a core. The geologist Richard Dixon Oldham noticed that P waves arriving on the far side of the planet from an earthquake were delayed. Something in the middle was slowing them down: a core.
1909: the bottom of the crust. The Croatian scientist Andrija Mohorovičić saw earthquake waves suddenly speed up a few dozen kilometers down. He’d found the boundary between crust and mantle, now called the Moho.
1914: how big the core is. Beno Gutenberg used wave timings to put the top of the core about 2,900 km below the surface, almost halfway to the center.
1926: the core is liquid. Harold Jeffreys combined earthquake data with measurements of how the solid Earth flexes under the Moon’s pull. The core behaved like it had almost no stiffness: it had to be liquid.
1936: a solid heart. The Danish seismologist Inge Lehmann spotted faint P waves arriving where they shouldn’t be. Her explanation: they were bouncing off a solid inner core inside the liquid one.

The shadow zone

There’s an elegant piece of evidence you can draw on a piece of paper. After a big earthquake:

S waves are recorded up to about 104° around the globe from the quake, and then they simply vanish. The liquid outer core blocks them, casting an S-wave “shadow” over the far side of the planet.
P waves do get through the core, but it bends them so much that a ring between about 104° and 140° from the quake gets almost no direct P waves.

The size of those shadows tells us how big the core is. The fact that S waves can’t cross it tells us it’s liquid.

Meet every layer, from crust to core →

Clue 3: Rocks from space

Some meteorites are made almost entirely of iron and nickel. They’re pieces of the metal cores of small planets that were smashed apart by collisions billions of years ago. They give us real samples of what a planetary core is made of.

There’s a twist, though: a core of pure iron and nickel would be heavier than Earth’s core actually is. So the core must also contain some lighter elements mixed in. Which ones is still an open question.

Clue 4: Squeezing iron in a diamond vise

Scientists can recreate the core’s crushing pressure in the lab. In a diamond anvil cell, a tiny speck of iron is pinched between the tips of two diamonds and heated with lasers while X-rays watch how it changes.

In 2013, a team at the European Synchrotron in France used this trick to measure when iron melts at extreme pressures. Their result put the temperature at the boundary of the inner core at about 6,000 °C, as hot as the surface of the Sun.

Clue 5: Earth’s magnetic field

A compass works because Earth is a giant magnet. The field comes from the liquid iron churning in the outer core, which acts like an electrical generator. The way the field slowly drifts and changes over the years gives scientists another window into the flowing metal 3,000 km below our feet.

What we still don’t know

Plenty! Scientists are still debating which light elements are in the core, how old the inner core is, and exactly how hot the center gets. Every big earthquake adds a few more clues.

Take the trip through every layer →