Deciphering the origin of seismic velocity heterogeneities in the mantle is crucial to understanding internal structures and processes at work in the Earth. The spin crossover in oxides of iron/magnesium, called ferropericlase, the second most abundant phase in the lower mantle, introduces unfamiliar effects on seismic velocities, which affect our fundamental understanding of the structure of the earth's mantle. This talk will overview our efforts using first principles calculations to elucidate the spin state of iron dissolved in mantle minerals and its broad implications for understanding geophysical processes.