We live on Earth's crust, but there are other layers beneath the crust. They are the mantle and the outer and inner core. In 1915, scientist Alfred Wegener said that about 200 million years ago, Earth once had a single landmass. Hot, molten magma under the surface of the crust pushed the plates apart at a crack in Earth's crust and, eventually, the landmass was split apart and continents were formed. Wegener's work led to the study of plate tectonics.