Nature teaches with different forces below the surface. She works with rough hands, using rock and gravity, faults and fissures, heat and chemistry. She builds the bedrock of the world. When men enter this underworld, they create a path through its dangers, going where there is no air, rockfall that is unpredictable, and gas seeping from the rock itself. Those who work in such conditions for years are transformed by the knowledge, they understand a world that does not work like the surface world, and learn how to live in an inhuman place.
For millennia people have explored such places inside the earth, and what they have brought up affects all of us. The ages of civilization are defined by the metals we get from the earth: the Stone Age, Copper Age, Bronze, Iron, Steel, and Silicon Age. Metals determine the tools we have, and the forces we can control. Almost everything we do and take for granted in our modern lives, comes from that underworld.