Geologists have learned about Earth's interior through two main types of evidence: direct evidence from rock samples and indirect evidence from seismic waves. Rock samples provide clues about conditions deep inside Earth, and sometimes volcanic eruptions blast rocks from over 100 km deep to the surface. Seismic waves from earthquakes reveal that Earth has layers like an onion, including a crust, mantle, and core that vary in size, composition, temperature, and pressure. The temperature increases rapidly within the first few tens of kilometers below the surface and continues rising towards the core. Pressure also increases greatly with depth.