Geologists have learned about Earth's interior structure using two main types of evidence: direct evidence from rock samples and indirect evidence from seismic waves. Rock samples provide clues about deep Earth conditions, as drills can reach over 12km deep and some rocks are blasted from over 100km deep. Seismic waves from earthquakes reveal the planet's structure based on their speed and paths through different layers. Geologists have determined Earth has three main layers - the crust, mantle, and core - which vary in size, composition, temperature, and pressure and surround each other like onion layers.