Overview: Twelve landmark steps to understanding the Earth's structure: Richard Oldham's theory of a liquid core, Inge Lehmann's conclusion that the core is solid iron, Alfred Wegener's premise of continental drift, Harry Hess's findings on seafloor spreading, plate tectonics, Leon Teisserenc de Bort's discovery of atmospheric layers, the theory of global warming, Victor Hess's studies of cosmic rays, Bernard Brunhes's assertion of magnetic field reversal, Charles Lyell's ideas on geological change, Bertram Boltwood's formulas for radiometric dating, and Milutin Milankovitch's ideas on periodic ice ages.