What is under the surface?

It is impossible to physically reach the center of the Earth.  So how do we know what lies under our feet?  Our students learn how to interpret what happens underground, starting at the surface and going down to the center of the Earth by measuring and interpreting acoustic signals such as earthquakes, or physical parameters such as the magnetic field, the gravity field, the geothermal heat flow and the Earth's electrical properties.  In this way it is possible to discover mineral beds, water reservoirs, underground energy sources such as oil and natural gas, and subterranean structures such as caves and buried buildings.  In this way we can better understand the structure of the planet we live on.