Nobody has been below 12 km
How We Know What Is Inside the Earth
The deepest hole ever drilled is a scratch. Everything else we know from listening.
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Step 1 of 5
Digging was never going to work.
The Kola Superdeep Borehole took twenty years to reach twelve kilometres, and stopped because the rock down there was hotter and softer than expected. On a globe the size of an apple, that is not through the skin.
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Step 2 of 5
So instead we listen to earthquakes.
A big earthquake shakes the entire planet, and the waves travel all the way through. Seismometers everywhere record when each wave arrives — and the arrival times depend on everything the wave passed through on its way.
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Step 3 of 5
Waves bend when the material changes.
A wave speeds up in denser rock and bends as it crosses a boundary — exactly as light bends entering water. Every bend in the arrival times marks a layer. That is how the boundaries were found, before anyone knew what was in them.
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Step 4 of 5
Then there is a shadow where nothing arrives.
The shadow is the discovery. It has a shape, and the shape is a sphere. Earthquakes make two kinds of wave. One kind can travel through liquid; the other cannot. On the far side of the planet from any large quake there is a wide band where only one of them ever shows up.
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Step 5 of 5
That shadow is the outer core, and it is liquid.
Its size gives the core's size. Its edge gives the depth. From there the whole layered picture follows: crust, mantle, liquid outer core, solid inner core. It was worked out by a Danish seismologist, Inge Lehmann, largely from arrival times she tabulated by hand.
That churning liquid metal is also why a compass works — which is its own page.
The short version
We know the core is liquid because one kind of earthquake wave never arrives on the far side of the planet.
Try it yourself
Tap one end of a long table and put your ear to the other. What you hear tells you about the wood in between, without looking at it.