Did you know If you saw a volcano erupting , you might think that Earth is filled with the hot liquid rock called magma. But in most places on Earth, you would never hit magma. Even if you drilled a mile into the ground, you would find only solid rock. So, which material is our planet made of? Because scientists can't peel away Earth's outer layers and look inside, they use evidence from the study of earthquakes to infer what the geosphere is like beneath its rocky surface. An earthquake causes vibrations that travel through the planet as waves. These waves move faster through solids and slower through liquids.

Machines called seismographs detect the waves from an earthquake almost anywhere on the planet. By analyzing how long it takes the waves made by an earthquake to travel through the planet, scientists can learn about Earth's inner structure.

The waves from the earthquake first move through the solid rock of the crust and mantle. When they enter a liquid layer, the waves slow down and bend, or refract. When the waves move from a liquid layer into another solid layer, they bend again. All of this bending of waves creates areas where no waves show up, called shadow zones. Observing these shadow zones led scientists to conclude that at least part of the Earth's interior is liquid.