Eli5: How do know about the Earths layers if we’ve barely scratched the surface ?

Question

Given it’s roughly 4,000 miles to the center of the corner, and according to resources, we’ve (humans) have only gotten about 18 miles deep. So how is it possible we know what’s beneath us?

In: 5

Answer 1

We can tell what’s inside by listening to seismic waves – massive sound-like waves produced by earthquakes – in a manner similar to how medical sonography is used to “see” what’s inside a human’s body without cutting it open. We also get a glimpse of the contents of the Earth when it gets ejected outside through volcanoes as lava.