Earth's lowermost mantle has thermal, chemical, and mineralogical complexities that require precise seismological characterization. Stacking, migration, and modeling of over 10,000 P and S waves that traverse the deep mantle under the Cocos plate resolve structures above the core-mantle boundary. A small -0.07 +/- 0.15% decrease of P wave velocity (Vp) is accompanied by a 1.5 +/- 0.5% increase ...