"... You can imagine the Earth as an enormous battery whose magnetic field is formed by the movement of molten metal (caused by the Earth's rotation) around the solid metal inner core. At present the positive terminal is at the , but in times past the orientation has switched so that the positive terminal is at the . Over the past 76 million years the Earth's magnetic field has reversed 171 times. No-one knows exactly why this happens, but at present the magnetic field is weakening a little each decade. Many researchers think that this presages another shift. Just how long before the reversal happens, however, is unknown."