Seismic waves change dramatically at the boundary between the Earth's mantle and the Earth's core. This boundary, called the Mohorovičić discontinuity (or Moho for short), is a significant transition zone where the density, composition, and physical properties of the Earth change drastically.

Here's why:

* Density: The core is significantly denser than the mantle, due to its composition of iron and nickel. This density change causes seismic waves to refract, or bend, as they pass from the mantle into the core.

* Composition: The core is mostly made up of iron and nickel, while the mantle is composed of silicate minerals. This compositional change affects the speed of seismic waves.

* Physical Properties: The core is also much hotter than the mantle, which affects the wave speed and causes changes in wave type. For example, P-waves (primary waves) are able to travel through liquids and solids, while S-waves (secondary waves) can only travel through solids. Because the outer core is liquid, S-waves are unable to pass through it.

These dramatic changes at the core-mantle boundary are crucial for understanding the Earth's internal structure and composition. Scientists use these changes in seismic wave behavior to map the Earth's interior and study its properties.