By studying earthquakes scientists have made images of the Earth's deep interior, far below the deepest drill holes of 10 km. As seismic waves travel through the Earth's interior on their way to distant recording stations, they are modified by the rock structures through which they pass.

By combining seismograms from many earthquakes recorded by many seismometers throughout the world, an image of the Earth's deep interior can be created. The imaging procedure, called seismic tomography, is very similar to the computer-aided tomography (CAT) scans in which doctors use X-rays to create an image the brain.

Modern global seismology has replaced old ideas of the Earth's interior.