system
Given the following post, respond as if you're commenting on Reddit:
human
How do we really know what's beneath the Earth's surface?

The deepest hole we've ever dug is the SG-3 branch of the Kola Superdeep Borehole, which penetrates 7.5 miles into the Earth's crust. As deep as this hole is, it only goes 0.002% of the way to the center of the Earth. How then can we know what is beyond the Earth's crust? How do we know what the Mantle consists of? How do we know what the outer and inner cores consist of? And how do we know the thickness/density of each?
gpt
We've discovered most of this by looking at seismic waves. By looking at the time it takes the vibrations from an earthquake to reach seismometers in different places, we can map out how quickly seismic waves travel through different regions of the earth's interior.

We can also do high-pressure lab experiments on predicted deep earth materials, and look at bits of deep earth material in volcanic lavas. For shallower depths, there are actually a few places on Earth's surface where upper mantle material is exposed. We also can understand some things by looking at how earth responds to adding or removing surface loads, like artificial reservoirs and the ice sheets that used to cover much of North America and Eurasia.

To answer your specific questions:

Besides the exposed upper mantle, geochemical theory predicts what minerals will be stable in an assemblage of certain elements at some pressure and temperature. We can see what elements are present in the deep earth by looking at certain igneous rocks. We also know the mantle *exists* because earth has too much gravity for a planet of its size that's made up of crustal material - Isaac Newton was the first to calculate this.

We know the core exists, and is mostly iron and nickel, because meteorites tell us earth should contain *a lot* of iron and we know it's not in the mantle.

We know there's a liquid outer core because we see that seismic S waves (which only travel through solids) don't propagate through it, and a solid inner core because seismic waves travel faster through it and bend.

We can find the thicknesses because seismic waves reflect off of boundaries, or discontinuities. Densities are constrained by seismic wave speeds and surface measurements of Earth's gravity.