An understanding of the numerous anomalies of water is closely linked to an
understanding of the phase diagram of the metastable non-crystalline states of ice. The discovery of high- (HDA) and low-density amorphous ice (LDA)  prompted the question whether this phenomenon of polyamorphism is connected to the occurrence of more than one supercooled liquid phase. In case of LDA the connection to the low-density liquid (LDL) was inferred from several experiments including the observation of a calorimetric glass-to-liquid
transition at 136 K . However, in case of HDA instead no calorimetric signature has been detected so far.
Experiments using differential scanning calorimetry (DSC) and dielectric spectroscopy show for the first time that HDA transforms into a liquid upon heating at ambient pressure . The glass-to-liquid transition upon heating is
evidenced by an endothermic step, which indicates that the liquid has a higher heat capacity than the glass due to an increase of molecular mobility, e.g., by unfreezing of translational motion. In our measurements the corresponding calorimetric signature occurs at 116 K. This glass transition can also be detected by dielectric spectroscopy via the appearance of an absorption peak centred at about 10–2 Hz. From the temperature dependent peak positions a relaxation map can be constructed. The good agreement between dielectric and calorimetric results convey for a clearer picture of water's vitrification phenomenon.
 O. Mishima, L. D. Calvert, E. Whalley, Nature 314, 76 (1985)
 G.P. Johari, A. Hallbrucker, E. Mayer, Nature 330, 552 (1987)
 K. Amann-Winkel et al., PNAS 110, 17720 (2013)