It is 3,950 miles (6,370 km) from the earth's surface to its center. The rock units and layers near the surface are understood from direct observation, core samples, and drilling projects. However, the depth of drill holes, and therefore, the direct observation of Earth materials at depth, is severely limited. Even the deepest drill holes (7.5 mi, 12 km) penetrate less than 0.2% of the distance to the earth's center. Thus, far more is known about the layers near the earth's surface, and scientists can only investigate the conditions within the earth's interior (density, temperature, composition, solid versus liquid phase, etc.) through more indirect means.

Geologists collect information about Earth's remote interior from several different sources. Some rocks found at the earth's surface, known as kimberlite and ophiolite, originate deep in the crust and mantle. Some meteorites are also believed to be representative of the rocks of the earth's...