Seismic waves are created by sudden movements of materials inside the Earth. Volcanic eruptions, earthquakes, explosions and avalanches are examples of circumstances that create such movementd and could create seismic waves. Scientists benefit from them because the seismic waves can be recorded by seismographs. Data from seismic waves is often used in order to better understand the inner structure of our planet. The waves can even be use to map details of that structure. One example of it is how they calculate de depth of Earth's major layers by studying the tame that some waves takes to travel down into the Earth and get back to the surface.