Magnetic poleshift reversal may cause the strength of the magnetic field to drop to zero.

"Through paleomagnetism, (the study of the magnetic properties of rocks), scientists now have solid proof that the earth's magnetic field, and thus the geomagnetic north and south poles, have reversed itself namy times in the past. Magnetic field polarity reversals are magnetic poleshifts. The geologic record also shows that the strength of the magnetic field varies widely in time, and fluctuates wildly during field reversals, sometimes dropping to zero gauss strength; ie: the field vanishes, disappears! It is also a proven fact that the magnetic poles wander, literally zig-zagging around its axis. "

http://www.crystalinks.com/poleshifts.html