Earth's inner core is dominated by iron, which can exist as a solid material in more than one crystallographic form. (This quality allows iron to combine with other elements to form alloys.) Iron's most stable form at room temperature is the body-centered cubic (bcc) structure. At extremely high pressures, it is stable in its hexagonal close-packed (hcp) phase. Of considerable debate, however, is iron's structure at the center of Earth. In a new study published in Geophysical Research Letters, Yang Sun and colleagues get one step closer to an answer.