Strong westerly winds occur in the stratosphere during winter and spring. These winds, the polar vortex, limit how much air is mixed between mid- and high-latitudes. We present a new view of the polar vortex mixing barrier in the Southern Hemisphere, revealing a frequent double-walled barrier with two distinct regions of weak mixing. This double-walled structure is expected to alter the spatial and temporal variation of trace gas concentrations (e.g. ozone) across the polar vortex.