Legendary anecdotes relate how Greco-Roman and Chinese societies may have first become aware of magnetism. For centuries it was thought to be a celestial power, but scientists learned through experimental investigations how magnetism is related to electrical currents. Its origin is now well understood at both large-scale and atomic levels. Deep within the Earth it has a liquid iron core, where fluid motions and electrical currents are driven by the planet’s internal heat and modified by its rotation. They form a self-sustaining generator for the Earth’s magnetic field. For centuries the field has been used by mariners to navigate across the world’s oceans. Their meticulous records now provide valuable information about changes in the field and related conditions in the Earth’s core. Surveys of the modern magnetic field – on land, at sea, and from satellites – provide information for geologists and geophysicists that helps them to explore the mineral wealth of unknown territories. The weak magnetism of rocks, acquired in the magnetic field at the time they formed, can be used to reconstruct the positions of continents in the distant geological past, thus enabling scientists to understand the planet’s history. In space, the Earth’s magnetic field interacts with the solar wind, a dangerous stream of electrically charged particles emitted by the Sun, and deflects it around the planet. Continuous records of changes in the magnetic field enable scientists to advise governments on measures to protect society against solar radiation, especially the violent electromagnetic events known as space weather–