The Earth's layers were discovered by looking at recordings of seismic waves generated by earthquakes. When an earthquake occurs, seismic waves travel throughout the earth. It is these seismic waves that cause the ground to shake near an earthquake. At great distances these waves can no longer be felt, but sensitive seismic instruments can still record them. Today's instruments are so sensitive that even moderate size earthquakes (magnitude 5) can be recorded on the other side of the world.

By looking at the recordings made by seismic instruments, scientists can figure out what the interior of the Earth looks like. This is much the same method as a CAT-scan used by doctors to image the brain.

The layers of the Earth were discovered during the first half of the 20th century after modern seismographs were invented. Scientists are still studying the details of the layers of the Earth in this way.

We can look at rocks on the surface and see that they are layered. We can also measure the larger-scale layers in the Earth by seeing the reflections and changes in direction that sound/earthquake waves make when hitting different layers in the Earth.