Direct information

For centuries geologists logically thinking that surface features must be the result of surface processes, have been searching for explanations all over the surface of the earth with their geopicks. Still, despite continued geological and economical activity, many remote areas of the earth are yet to be explored in any kind of detail. In the last two decades, aerial and satellite photography and other remote sensing techniques have provided us with a more global view of the geologic surface patterns. 

Logically, most of the detailed information we possess involves the uppermost layers, and the deeper we go below the surface, the poorer our information becomes. Mines reach down to 3 miles. Drilling, especially for oil and gas, has rarely exceeded 20 miles in depth. Despite all of this activity carried out since Stone Age times, we have never physically penetrated beyond the earth's uppermost layers. 

Some direct information on deeper conditions is available from volcanos, whose lava usually originate from only a few miles below the surface. However, these molten materials change markedly during their upward journey to the surface, both by losing gases and by being altered compositionally. 

Indirect information

There is virtually no direct information from beyond 50 miles below the surface. Limited by our inability to see and directly test the materials inside of the earth, we are dependent on indirect information derived from earthquake wave behavior. Seismic techniques have been shown to be especially useful for investigation of the shallow as well as the deeper layers of the earth. Thousands of natural earthquakes occur annually, others are man-made and are basic tools of seismic exploration.