How did scientists determine earth has layers?

The seismologists have determined the distinct boundaries that are present within the interior of the earth. These have been identified by studying the behavior of seismic waves. These waves include the primary and the secondary waves, commonly represented as P and S waves.

The P waves were found to get deviate as it enters the outer core from the mantle, whereas the S waves get dissolve as it reaches the core-mantle boundary. These data helped the seismologists to understand the changing composition and density of the earth at various depths. This is how the internal layers of the earth were determined.