Earth's magnetic field looks like strands of spaghetti in these two supercomputer simulations from researchers at Los Alamos National Laboratory. The image at left shows the magnetic field in a stable state. The image at right shows it in the middle of a reversal, when magnetic "north" becomes magnetic "south" and vice versa. Such reversals happen on average every few hundred thousand years, with the last about 780,000 years ago. The field grows considerably weaker during a reversal, allowing more space radiation to reach Earth's surface. [Gary A. Glatzmaier/LANL/DOE]