Geomagnetic reversal
This has happened 183 times over the last 83 million years, averaging about two or three times per million years.  Before a change of magnetic field, the Earth's magnetic field becomes weaker and moves around, like a spinning top would before it falls.  Scientists know this as a result of studies of magnetism on the sea floor, near the mid-Atlantic ridge.  The lava slowly moves out of this crevasse (gap in the sea floor) and then it cools with its iron oxide molecules all pointing in the new direction of the Earth's magnetic field.  We can look at the history of this magnetic field today to look back at the many flips in the past.