evidence: Earth's magnetic north and south poles have flip-flopped many times in our planet's history—most recently, around 780,000 years ago. Geophysicists who study the magnetic field have long thought that the poles may be getting ready to switch again, and based on new data, it might happen earlier than anyone anticipated. [...] Geophysicists do not yet fully understand the process of geomagnetic reversals, but they agree that our planet's field is like a dipole magnet. Earth's center consists of an inner core of solid iron and an outer core of liquid iron, a strong electrical conductor. The liquid iron in the outer core is buoyant, and as it heats near the inner core, it rises, cools off and then sinks. Earth's rotation twists this moving iron liquid and generates a self-perpetuating magnetic field with north and south poles. [...] It is hard to know how a geomagnetic reversal would impact our modern-day civilization, but it is unlikely to spell disaster. Although the field provides essential protection from the sun's powerful radiation, fossil records reveal no mass extinctions or increased radiation damage during past reversals. A flip could possibly interfere with power grids and communications systems—external magnetic field disturbances have burned out transformers and caused blackouts in the past. But Glatzmaier is not worried. "A thousand years from now we probably won't have power lines," he says. "We'll have advanced so much that we'll almost certainly have the technology to cope with a magnetic-field reversal."