A stamped handle from Ramat Rahel. Such fragments are an archive of the strength of the Earth's magnetic field at the time they were made. A collection of clay storage jar handles from ancient Judah - modern-day Jerusalem - is providing fresh insight into fluctuations in the Earth's magnetic field. It's long been known that the magnetic field - extending from the centre of the planet out to about 65,000 kilometres above the surface - changes over time. The north and south poles shift to and fro over centuries, and every few hundred thousand years they even swap places. Since the development of pottery, these movements have been inadvertently encoded in human artefacts. Clay contains small amounts of a mineral known as magnetite. When fired in a kiln, the magnetite transforms into a permanent record of the intensity and alignment of the magnetic field at that moment. Today, movement in the magnetic fields are made accurately and easily, but recreating past shifts - a field known as