In proton exchange membrane fuel cells, the interior porosity of the standard high surface area carbon (HSC) supports anchors and shields catalysts, offering benefits in performance and durability. Yet, these carbons also add mass transport resistance. This delicate tradeoff relies on their interior diffusion pathways, which are difficult to fully characterize and remain poorly understood. Here, the multiscale morphology of HSCs is reported using full-range electron tomography, resolving features down to single carbon planes. It is found that the supports typically feature a core-shell morphology with large central pores and compact shell in which pores are slit-shaped and sub-nm in size, while entry points are 7-8 & Aring; in diameter and are rarely in close proximity to Pt catalysts. This remarkably resolved structural landscape reveals that O-2 diffusion pathways in HSCs are narrower and longer than previously assumed, indicating the critical value of the carbon support redesign for optimizing cell performance.