Supercooled water may offer clues to the anomalous properties of its normal liquid state. The supercooled state also shows anomalous thermodynamic and transport properties at low temperatures. Although there are several theoretical explanations for this behaviour, no consensus has emerged. Some theories preclude the existence of the supercooled liquid below an apparent thermodynamic singularity at 228 K (refs 2, 7, 9); others are consistent with a continuous region of metastability from the melting point at 273 K to the glass transition temperature at 136 K (refs 6, 8, 13). But the data needed to distinguish between these possibilities have not yet been forthcoming. Here we determine the diffusivity of amorphous ice by studying isotope intermixing in films less than 500 nanometres thick. The magnitude and temperature dependence of the diffusivity is consistent with the idea that the amorphous solid water melts into a deeply meta-stable extension of normal liquid water before crystallizing at 160 K. This argues against the idea of a singularity in the supercooled regime at ambient pressure.
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