A team of geologists has found for the first time evidence that two ancient, continent-sized, ultrahot structures hidden beneath the Earth have shaped the planet's magnetic field for the past 265 million years. Within them, the mantle material is hotter, denser, and chemically different from the surrounding material. After another 10 years of research, they now point to them directly as structures capable of modifying Earth's magnetic field. This inequality contributes to the magnetic field taking on the irregular shape we observe today. The geodynamo simulations also revealed that some parts of the magnetic field have remained relatively stable for hundreds of millions of years, while others have changed remarkably.