Some years ago, series of magnetized 'stripes' running parallel to mid-ocean ridges were found that record the history of the Earth's magnetic field. They show that it reverses direction quite often (in the geological sense of the word) and is even quite variable between reversals.
    Current theory regarding the source of the Earth's magnetic field has it that the field arises from electrical currents deep within the Earth. Motivated by a professor (Dr. Detenbeck at UVM) I had while doing my BS degree, I read several vintage papers on the subject which essentially treated the Earth as a set of coupled electrical dynamos. The resulting differential equations turn out to be quite non-linear and have to be integrated via numerical methods. This I wrote a computer program to do.
    Each of the dynamos has a set of parameters (inductances, resistances, moments of inertia etc.) associated with it, and the behavior of their magnetic fields (and therefore the Earth's field) varies wildly as these parameters are tweaked. However one thing is clear: even a simple coupled-dynamo model can qualitatively account for the magnetic field reversals recorded on the ancient 'magnetic tape' of the seafloor.