To date, approaches seeking a definitive link between colloidal suspension microstructure and the corresponding viscosity predict that for dense suspensions, hard-sphere systems possess the lowest-attainable viscosity. Interparticle interactions are thought to increase suspension viscosity because of the additional energy that is required to overcome these interactions to sustain flow. Here we report our recent findings on the ability of weak depletion attractions to generate additional free volume within dense suspensions that then enhance particle diffusivities and result in a viscosity that is lower than the hard-sphere value. The minimum achievable viscosity relative to the hard-sphere value grows with increasing volume fraction in a continuous fashion as the glass-transition volume fractions are approached. Calculations using a recently developed model that incorporates the ability of thermal motions to relax particle localizations in cages within a mode coupling framework qualitatively capture our experimental findings. The study highlights the presence of localization barriers in dense suspensions above volume fractions of similar to 0.40-0.45 that grow stronger with volume fractions up to and beyond the glass-transition volume fractions and challenges long-held notions that hard spheres possess the lowest-attainable viscosities at these dense volume fractions.