system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How do geologists know whats located below Earth's crust?
gpt
When you ring a bell sound waves travel through it, bouncing from one side to the other in the hollow cavity creating that ringing echo.

When an earthquake occurs, seismic waves do something similar with the entire planet - bouncing around the interior and deflecting and changing as they travel through different materials.

Using detector stations on different parts of the planet you can detect a few things about the earth just from how the waves travel through it:

Waves that travel *near* the center are curved and distorted as if they’ve moved through a metallic liquid for a while, and then back into rock.

Waves bounce off the very center, as if it’s a giant sphere of solid metal.

Waves that don’t travel near the center aren’t distorted, suggesting they’re only traveling through rock.

By measuring these waves and the paths they take, you can draw up a diagram of approximately where the different material layers in the earth’s innards are located.

Measurements of earth’s density and magnetic field support these models of a liquid metal outer core and solid metal core.