system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 : how do we know what's under the crust?

How to we know what the layers of earth are? I've learnt in primary school that underneath the crust is the mantle, then the outer and inner core, but how can we prove this if we have never dug below the crust?
gpt
By looking at the stuff that bubbles up through features like volcanoes.

We can also use data from seismic measurements around the world. As the energy produced by earthquakes moves it will deflect as it goes through areas of different density. By collecting data from all over the world we can get a pretty good idea of the depths of different layers.

We can also measure meteorites in our solar system older than the earth's crust to get an idea of the likely composition of the earth.