Seismic tomography models reveal that both upwelling and downwelling mantle flow is deflected at multiple depths near to 1000 km in the mid-mantle. However, there are no candidate mineral physics changes in the mid-mantle which readily explain this deflection, and thus the causative mechanisms remain controversial. Compositional and viscosity changes have been proposed, but a lack of global observations of seismic properties at this depth precludes any further investigations. Here, I will present a recent global study of regional-scale mid-mantle reflectors, from 800 to 1350 km depth in the mid-mantle. Reflectors across the full depth range are detected globally, as are regions in which reflectors are absent, indicating widespread heterogeneity. Their seismic properties are found to be characterised according to the background seismic velocity (i.e. fast, slow, or neutral, with respect to a global average), and thus mantle flow patterns. The observations are interpreted in terms of both seismic tomography and mantle dynamic models. They ultimately inform regarding configuration and composition of mantle structures including slabs, thermochemical piles, and reservoirs, as well as the regionally-diverse styles of mantle flow and mixing.