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.