The presence of an intermittent layer at the base of Earth’s mantle with a maximum thickness near 40 kilometers and a compressional wave velocity depressed by similar to 10 percent compared with that of the overlying mantle is most simply explained as the result of partial melt at this depth. Both the sharp upper boundary of this layer (<10 kilometers wide) and the apparent correlation with deep mantle upwelling are consistent with the presence of liquid in the lowermost mantle, implying that the bottom of the thermal boundary layer at the base of the mantle may lie above its eutectic temperature. Such a partially molten zone would be expected to have enhanced thermal and chemical transport properties and may provide constraints on the geotherm and lateral variations in lowermost mantle temperature or mineralogy.