  During the Leidenfrost effect, a thin insulating vapor layer separates an
evaporating liquid from a hot solid. Here we demonstrate 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 $\approx 140^{\circ}$C, corresponding to an average vapor layer
thickness of 10-20$\mu$m.
