In the early part of the 20th century, geologists studied the vibrations (seismic waves) generated by earthquakes to learn more about the structure of the earth's interior. They discovered that it is made up of these distinct layers: the crust, the mantle, and the core.
Image credits: Animation adapted from illustrations by Jane Russell from Jacquelyne Kious and Robert Tilling, This Dynamic Earth, USGS, p.8, and from Eugene C. Robertson, The Interior of the Earth, USGS.
