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