Hydrogen embrittlement (HE) causes sudden, costly failures of metal components across a wide range of industries. Yet, despite over a century of research, the physical mechanisms of HE are too poorly understood to predict HE-induced failures with confidence. We use non-destructive, synchrotron-based techniques to investigate the relationship between the crystallographic character of grain boundaries and their susceptibility to hydrogen-assisted fracture in a nickel superalloy. Our data leads us to identify a class of grain boundaries with striking resistance to hydrogen-assisted crack propagation: boundaries with low-index planes (BLIPs). BLIPs are boundaries where at least one of the neighboring grains has a low Miller index facet—{001}, {011}, or {111}—along the grain boundary plane. These boundaries deflect propagating cracks, toughening the material and improving its HE-resistance. Our finding paves the way to improved predictions of HE based on the density and distribution of BLIPs in metal microstructures. See the associated publications: John P. Hanson, Akbar Bagri, Jonathan Lind, Peter Kenesei, Robert M. Suter, Silvija Gradecak, Michael J. Demkowicz, "Crystallographic character of grain boundaries resistant to hydrogen-assisted fracture in Ni-base alloy 725", Nature Communications 9:338 (2018). DOI: 10.1038/s41467-018-05549-y Akbar Bagri, John P. Hanson, Jonathan Lind, Peter Kenesei, Robert M. Suter, Silvija Gradecak, Michael J. Demkowicz, "Measuring grain boundary character distributions in Ni-base alloy 725 using high-energy diffraction microscopy", Metallurgical and Materials Transactions A 48, 354-361 (2017). DOI: 10.1007/s11661-016-3831-x