Red and blue regions depict zones where seismic waves propagate slower or faster than average, respectively. Distant earthquakes (e.g., the red star) send seismic energy throughout the planet, which traverses anomalous structure and brings information about Earth’s internal structure to the planet’s surface. The large red region beneath the Pacific Ocean sits atop the hot molten iron core (orange ball), is best explained as chemically distinct from the rest of the mantle. Credit: AAAS/Science