Earth's magnetic field
A geomagnetic reversal is when the north magnetic pole and south magnetic pole trade places.  This has happened a few times in the past few million years.  The magnetic reversal happens after the strength of the field reaches zero.  When the strength begins to increase again, it will increase in the opposite direction, causing a reversal of the magnetic poles.  The time it takes the magnetic field to undergo a reversal is unknown, but can last up to ten thousand years.