How far can you follow a compass needle? As far as the north magnetic pole where the needle starts spinning wildly? Compass needles align with magnetic field lines; and on the precise spot of magnetic north, those field lines are vertical. So just tilt your compass 90 degrees and you can continue your journey: either down to the molten iron dynamo surrounding Earth's core; or up. But up to where? The answer: to everywhere. And today that's exactly where we're going to go.
