ScienceBlogs.com: It's fairly common knowledge that the Earth's magnetic field periodically reverses its polarity. At the moment, magnetic field lines run from the South Pole to the North Pole, and point up in the southern hemisphere and down in the northern hemisphere. But at many points in the past, the field lines (and compasses, if they'd been invented) pointed south were directed upward in the northern hemisphere and downward in the southern hemisphere.