The interior of the Earth is separated into three chemically distinct parts: the crust, mantle, and core. At the present time only the outer layer of the Earth, a relatively wafer-thin section, has been directly studied.

Via Wikipedia

The deepest boreholes have yet to reach even one percent of the 3,959 mile (6,371 km) distance to the center of the Earth. Instead, our best information about the Earth’s interior are the waves generated during earthquakes.

The speed with which earthquake waves travel depends on the temperature, pressure, and composition of the rocks they penetrate. Wave measurements indicate that the physical characteristics of the Earth’s layers change dramatically in two regions. The discontinuities mark the delineation between the crust, mantle, and core.