Reciprocity XXVII, #1 (Spring, 1998), p. 9.
Very little is actually known about the Earth's interior. Actual research is limited to what is pulled up from a scant few miles of the crust, by deep mines and drilling rigs. Volcanoes provide some additional insight as to the existence of a molten plastic-like layer between the crust and mantle known as the asthenosphere. However, the bulk of data beyond this point comes from the distant echoes of earthquakes, and the seismographic machines that plot their deviations as they traverse the depths of the Earth's interior.