Swedish researchers have presented evidence in Science to support their new theory about the structure of the earth’s core. The findings may be of significance for our understanding of the cooling down of the earth, and of the stability of the earth’s magnetic field. It has long been known that the inner core of the earth, a sphere consisting of a solid mass with a radius of about 1,200 km, is mainly made up of iron. However, seismic observations have shown that elastic waves pass more rapidly through this core in directions that are parallel to the earth’s axis of rotation than in directions parallel to the equator-a phenomenon that has not been previously explained.