Scientists have known for a long time now that the Earth's magnetic field unexpectedly reverses polarity over long periods of time, but how often this happens was still a mystery. Brad Clement, a geologist funded by the National Science Foundation, has concluded that the reversal seems to happen every 7,000 years, on average. He gathered this data by analyzing deep-ocean sediment cores. One surprise is that the variation seems to alter with latitude - the directional change takes half as long at low-latitude sites as it does at mid- to high-latitude sites.