Seismologists are geologists who specialize in studying seismic waves in the Earth to understand Earth structure. Earthquakes produce seismic waves. Different materials and different layers of the Earth have properties that cause changes in the way seismic waves travel. Using these differences, seismologists can get a picture of the structure of the Earth, from the core to the surface. They may apply seismology on many scales, from studying the layers of sediment on the seafloor to studying the whole Earth. Seismologists can use seismic waves naturally generated by earthquakes or generate their own through devices such as air guns.