The interior structure of the Earth, similar to the other terrestrial planets, is layered. These layers can either be defined by their chemical or rheological properties. The Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. Scientific understanding of Earth's internal structure is based on extrapolations of physical evidence which has come from the first few kilometres of Earth's surface, samples brought to the surface from deeper depths by volcanic activity and analysis of seismic waves that pass through it.