Brunhes observed that in some rocks, crystals magnetized in directions opposite to Earth's magnetic field. This was initially ignored but then suggested that Earth's magnetic field fluctuated and even changed polarity completely. This was verified with magma sediment on the seafloor which showed both evidence of seafloor spreading and of irregular reversals of the magnetic field.
A visual chronology of Asimov's ~1500 scientific inventions and discoveries.
