system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 How did scientists find out what layers exist under the Earth's crust? How did they determine that Earth has a solid core?
gpt
Most of this is figured out through seismic surveys. You make a loud sound somewhere and then measure how the sound waves travel through the Earth. This is exactly how ultrasonic surveys work in humans. The sound waves will experience different levels of refraction or even reflection as they hit layers where the density changes. We can measure this and figure out the density of all the rock. It turns out that the core of the Earth is very solid compared to the material above it. So similar to how sound bounces between the walls in a room but is muffled outside the room any loud sounds on Earth is bounced back from the core but is very muffled on the other side of the Earth.