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
-Milutin Milankovitch's ideas on periodic ice ages
