We have mapped shear velocity variations near the base of the mantle with ScS-S differential travel times. Beneath the North Atlantic, there is a systematic variation in velocity over a distance of 1500 km or more, with a pronounced low velocity anomaly centered roughly beneath Madeira island. Beneath the northern Indian Ocean, there is no similar long-wavelength anomaly. The average ScS-S residual beneath the Indian Ocean is 3.6¿0.7 s more negative than the Atlantic residual, indicating faster average velocities near the base of the mantle.