Understanding of the Earth's structure has increased dramatically over the last 200 years. This program illustrates 12 landmark steps along the way: Richard Oldham's theory of 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.