the movement of molten iron-itself magnetized billions of years ago the sun or some other celestial body-thousands of miles beneath the planet's surface. As the liquid metal rises, it gradually cools and begins sinking back toward the Earth's core, creating "eddies" some 100 miles in diameter. There may be as many as 50 of them. The rotation of the Earth on its axis makes most (but not all) of the eddies point either north or south. "The net direction of the magnetic field,"...