The potential to enable unprecedented performance, durability, and safety has created the impetus to develop bulk-scale all-solid-state batteries employing metallic Li as the negative electrode. Owing to its low density, low electronegativity and high specific capacity, Li metal is the most attractive negative electrode. However, failure caused by the formation of dendrites has limited the widespread use of rechargeable batteries using metallic Li negative electrodes coupled with liquid electrolytes. One approach to mitigate the formation of dendrites involves the use of a solid electrolyte to physically stabilize the Li–electrolyte interface while allowing the facile transport of Li-ions. Though in principle this approach should work, it has been observed that at high Li deposition rates Li metal can propagate through relatively hard ceramic electrolytes and Li dendrite formation causing short circuit has been reported. Why this occurs is poorly understood, emphasizing the need to close the knowledge gap and facilitate the development of advanced batteries employing solid electrolytes. Here, through precise microstructural control, striking electron microscopy, and high-resolution surface spectroscopy, we directly observed for the first time the propagation of Li metal through a promising polycrystalline solid electrolyte based on the garnet mineral structure (Li6.25Al0.25La3Zr2O12). Moreover, we observed that Li preferentially deposits along grain boundaries (intergranularly). These results offer insight into the electrochemical-mechanical phenomena that govern the stability of the metallic Li–polycrystalline solid electrolyte interface and are essential to the maturation of solid-state batteries.