The deepest hole dug by humans is 12 km deep, how do we know what is there 2900 km deep?
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Every once in a while the Earth spits out the content of its core, so we take a look.
Also our physics says how rocks behave when there is million tons of other rocks on top of them: they melt under the pressure.
We use seismic waves from explosions and earthquakes as we can tell how they travel through different mediums. Also, we know what it would take to generate a magnetic field such as the one we have. If the earths core were made of sand it wouldn’t generate a magnetic field or if it were made of different metals it would create a weaker field.
Simplified and partially misleading, but scientists just take a guess.
Partially misleading because before they guess they look at all the things we can see from the surface. Magnetic fields, volcano contents, seismic reactions, geologic surveys, strength of gravity, etc. And quite a few scientists talk about it for a while before they come up with a best guess.
But even nowadays the guess is occasionally updated! In the last 2 years, the common theory is making room for a slightly different core than it was before because of new data.
Henry Cavendish performed an experiment in 1798 to measure the average density of earth. He did measure the gravitational attraction between lead spheres and from it he could calculate the mass of earth. You can get the gravitational constant G from it even if he did not calculate it.
This is the [https://en.wikipedia.org/wiki/Cavendish_experiment](https://en.wikipedia.org/wiki/Cavendish_experiment) and the result was the average density was 5.448 times the density of water.
This is higher than the density of the rocks we find in the crust and the lava from the mantle released in volcanoes. For example, Granite is very common to have a density of around 2.75x water. So the conclusion is that there has to be lot of denser metal in earth.
With the help of seismic waves in earthquakes, you have two types of waves. P-wave is a pressure wave-like sound in air. S-waves is shear wave that is a vibration side to side perpendicular to the direction the wave travel. Take a rope, lay it on the ground, and jank it to the side and you get a S wave. The S waved can’t travel in liquid or gases but can move in solids. A wave that moves from solids to a liquid and vice versa will slightly change in direction
IF you measure the seismic waver from earthquakes in different locations of earth the result is [https://akm-img-a-in.tosshub.com/indiatoday/images/story/201602/seismic-waves-650_042715041742.jpg](https://akm-img-a-in.tosshub.com/indiatoday/images/story/201602/seismic-waves-650_042715041742.jpg)So by just observing the waves we can determine that there is an outer solid mantle around a liquid outer core and a solid inner core. This is fundamentally the same way ultrasound is used to image an unborn child.
What metal is in there was determined by meteorites that were formed in the solar system the same time as the earth. The ones that contain lots of metal are primarily made of Iron and Nickle. We alos know how the elements are produced in starting and exploding supernovas in [https://en.wikipedia.org/wiki/Stellar_nucleosynthesis](https://en.wikipedia.org/wiki/Stellar_nucleosynthesis) and the dense metal that is primary produce are iron and nickel. We can observe clouds from supernovas in out galaxy and see planetary systems getting formed. With [https://en.wikipedia.org/wiki/Spectroscopy](https://en.wikipedia.org/wiki/Spectroscopy) we can determine their composition and it is lots of iron and nickel. So the reasonable conclusion is that the core of earth is the same, primary iron and nickel.
So there is no need to reach the core of earth to have a very good idea of what it is made of.
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