Much of what we know about Earth's interior comes from seismic waves generated by earthquakes. Seismic waves are waves of energy that travel through the Earth. Earthquakes produce three basic types of seismic waves: P waves, S waves, and surface waves. Each type has a different type of motion and travels through the Earth at different speeds. Scientists use seismographs to measure these waves, which helps them understand the structure and composition of Earth’s layers.