While we have sent probes billions of kilometers into interstellar space, humans have barely scratched the surface of our own planet, not even making it through the thin crust. Information about Earth's deep interior comes mainly from geophysics and is at a premium. We know it consists of a solid crust, a rocky mantle, a liquid outer core, and a solid inner core. But what precisely goes on in each layer – and between them – is a mystery . Now our research uses our planet's magnetism to cast