How did scientists discover the internal structure of the earth?

Hint: Earth is made up of three different layers namely the core, the crust, and, the mantle. The outside layer of the earth is made up of solid rock like granite and basalt.

Complete answer:
In 1909, a Croatian seismologist Andrija Mohorovicic discovered the internal structure of the earth.
By using seismic waves, scientists discovered the internal structure of the earth. Earth’s interior comes from the seismic waves. The waves that are caused by earthquakes are called seismic waves. Body waves and surface waves are the two main types of seismic waves. The body waves travel through the Earth’s interior surface in all directions.
The common seismic waves are S waves and P waves. S waves will move only through the solids and will have more energy. P waves will move through all the three states of the matter and will have less energy. S waves are slower when compared to P waves.
Thus the seismic waves when passed through the different types of material at different speeds, scientists can know about the Earth’s internal structure by studying the seismograms.
We use the earthquake detectors to find out if there is any difference in the speed. Also, we know that P waves are quite slower in the liquid than in the solid. So by using these differences and by finding the difference in the speed, whenever a wave refracts from one state to the other state, we can discover what is inside the earth.

Note: 1. The thickest layer is the crust and the inner core is solid.
2. The only planet that was not named after a Roman god or Roman goddess or Greek is Earth.