The document describes the layers that make up the interior of the Earth. It is divided into four main layers from outermost to innermost: the crust, mantle, outer core, and inner core. Scientists use seismic waves from earthquakes to determine the composition and structure of these layers, since they cannot directly observe the interior. The waves travel at different speeds through solids and liquids in the Earth, providing clues about what materials exist in its core and where transitions between layers occur.