Earth's magnetic field is continuously changing. Magnetic north wanders, and every few hundred thousand years the polarity gradually flips, so that a compass would point south instead of north.
This is the second of three videos:
Magnetic field: an introduction: http://www.youtube.com/watch?v=P7nhgX0ppek
Magnetic field: why it matters
Magnetic field: learning more with Swarm http://youtu.be/Wn-BV9Zp8Pk
