Most of us take it for granted that compasses point north. Sailors have relied on the earth's magnetic field to navigate for thousands of years. Birds and other magnetically sensitive animals have done so for considerably longer. Strangely enough, however, the planet's magnetic poles have not always been oriented as they are today.

Minerals that record past orientations of the earth's magnetic field reveal that it has flipped from north to south and back again hundreds of times during the planet's 4.5-billion-year history. But a switch has not occurred for 780,000 years--considerably longer than the average time between reversals, about 250,000 years. What is more, the primary geomagnetic field has lessened by nearly 10 percent since it was first measured in the 1830s. That is about 20 times faster than the field would decline naturally were it to lose its power source. Could another reversal be on its way?