evidence: From animal migrations to human communications, a reversal of Earth’s magnetic poles could seriously mess with life as we know it. Many facets of our lives depend on the Earth’s magnetic field, anchored by the North and South poles, from the electrical grid that powers our computers to the satellites that let us watch TV. Turtles and other creatures navigate with it. But as Alanna Mitchell shows in her new book, The Spinning Magnet, it wasn’t always that way. Indeed, as little—in geologic time, anyway—as 780,000 years ago, the poles reversed. It may be about to happen again, some scientists believe, with potentially disastrous results for life on Earth. (Why it's not time to panic yet about the magnetic field flip.)