In many ways, scientists know more about the structure of the universe and the workings of microscopic cells than they do about the ground beneath their feet. With a telescope, an astronomer can see stars at the farthest edges of the universe. With a microscope, a biologist can see the inner structure of cells. But there is no instrument that allows geologists to look deep inside Earth. The reason is both simple and profound: light does not travel through most rocks.

Though scientists cannot see into Earth, in a sense they can listen to it. The waves from earthquakes travel through Earth in a manner similar to the way that sound waves travel to our ears. These waves vibrate within Earth and are recorded at Earth's surface. Scientists are able to interpret these waves to learn about the structure and composition of Earth's interior. Scientists have also drilled holes to a depth of about 12 kilometers to collect data on the uppermost portion of Earth's interior. Figure 1 shows one such drilling effort.