Introduction: Mountain climbers summit the tallest peaks on earth. Scuba divers explore the depths of ocean reefs. Yet, we all live on just the outermost shell of our planet, the crust. Most of the earth's volume is contained deep below our feet, but is essential to life here on its surface. Convection currents drive plate tectonics deep within the earth. A liquid metal outer core generates earth's protective magnetic field. Understanding the internal structure of our planet is essential to understanding life on earth.