Vertically-polarized S-waves (SV) attenuate rapidly as they diffract around the Earth's core, whereas horizontally-polarized S-waves (SH) propagate to large distances in the core shadow zone. The amplitude decay of diffracted SV signals is so strong that few studies have been made of these phases, despite their acute sensitivity to velocity structure just above the core-mantle boundary. We analyze SV signals in the penumbra of the core's shadow, finding systematic waveform complexities indicative of local stratification and apparent anisotropy at the base of the mantle, with implications for the dynamic processes in this internal boundary layer. ¿ American Geophysical Union 1991