The document discusses the Earth's internal structure and how it is studied using seismic waves. There are three main layers within Earth - the core, mantle, and crust. The core is the innermost layer and makes up about 1/3 of Earth's mass. The mantle is the middle layer and comprises about 2/3 of Earth's mass. The crust is the outermost layer and is a very small fraction of Earth's total mass. Seismic waves generated by earthquakes travel through Earth at different speeds depending on the material, allowing scientists to map its internal structure.