During the Leidenfrost effect, a thin insulating vapor layer separates an evaporating liquid from a hot solid. Although studied for more than 3 centuries, the temperatures at which these vapor layers form or fail are not well understood. Here we demonstrate that there are 2 characteristic temperatures to consider, and that Leidenfrost vapor layers can be sustained at much lower temperatures than those required for formation. Using a high-speed electrical technique to measure the thickness of water vapor layers over smooth, metallic surfaces, we find that the explosive failure point is nearly independent of material and fluid properties, suggesting a purely hydrodynamic mechanism determines this threshold. For water vapor layers of several millimeters in size, the minimum temperature for stability is ≈140°C, corresponding to an average vapor layer thickness of 10–20 microns. Additionally, the experiments are supported by our recent simulations which reveal how the hydrodynamic instability and failure occur.