Humans have been all over the Earth. We've conquered the lands, flown through the air and dived to the deepest trenches in the ocean. We've even been to the Moon. But we've never been to the planet's core.
We haven't even come close. The central point of the Earth is over 6,000km down, and even the outermost part of the core is nearly 3,000 km below our feet. The deepest hole we've ever created on the surface is the Kola Superdeep Borehole in Russia, and it only goes down a pitiful 12.3 km.
All the familiar events on Earth also happen close to the surface. The lava that spews from volcanoes first melts just a few hundred kilometres down. Even diamonds, which need extreme heat and pressure to form, originate in rocks less than 500km deep.
What's down below all that is shrouded in mystery. It seems unfathomable. And yet, we know a surprising amount about the core. We even have some idea about how it formed billions of years ago - all without a single physical sample. This is how the core was revealed.
One good way to start is to think about the mass of the Earth, says Simon Redfern of the University of Cambridge in the UK.
We can estimate Earth's mass by observing the effect of the planet's gravity on objects at the surface. It turns out that the mass of the Earth is 5.9 sextillion tonnes: that's 59 followed by 20 zeroes.
There's no sign of anything that massive at the surface.
"The density of the material at the Earth's surface is much lower than the average density of the whole Earth, so that tells us there's something much denser," says Redfern. "That's the first thing."
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