How did scientists discover the internal structure of the earth?

The two most common types of seismic waves are S and P waves. S waves are slower than P waves and have greater energy, but they can only flow through solids; on the other hand, P waves are faster and have less energy, but they can flow through all three states of matter. Since we have earthquake detectors, we use the detectors to determine whether there is a difference in speed; P waves move more slowly through liquids than through solids. By calculating the difference in speed when a wave refracts to a different state, we can determine what is inside the earth: the mantle was solid, the outer core was liquid, and the inner core was solid.

Through the analysis of seismic waves produced by earthquakes and explosions, scientists were able to uncover details about the composition and layering of the Earth's interior by measuring the time it took for these waves to travel through the Earth.