In 1906 French physicist Bernard Brunhes (1867–1910) found rocks with magnetic fields oriented opposite to that of Earth’s magnetic field. He proposed that those rocks had been laid down at a time when Earth’s magnetic field was oriented opposite to the way it is today. Brunhes’s idea received support from the research of Japanese geophysicist Motonori Matuyama (1884–1958), who in 1929 studied ancient rocks and determined that Earth’s magnetic field had flipped its orientation a number of times over the history of our planet. Today, studies of both rock and the fossilized microorganisms imbedded in the rock show that at least nine reversals of Earth’s magnetic field orientation have occurred over the past 3.6 million years.

The exact cause of the polarity reversal of Earth’s magnetic field is still unknown. Current hypotheses suggest that the reversal is caused by Earth’s internal processes, rather than external influences like solar activity.