Most of the lower mantle is relatively simple in terms of seismic structure. Lateral variations are weaker compared to its bottom part and to the upper mantle and seismic velocities increase smoothly with depth. To a first order, these features appear consistent with the adiabatic compression of a homogeneous medium. Here we show, instead, that seismic data require a change of chemical and/or thermal state with depth below ∼1600 km, if the effect of the spin-transition of Fe2+ in ferropericlase on the elastic signature of a typical lower mantle assemblage is taken into account. The inferred thermochemical structure helps to reconcile geochemical and geophysical observations and has profound implications on the dynamical evolution of our planet.