Microthermometric measurements of a synthetic high-density (984 kg m−3) water inclusion in quartz revealed that only part of the super-cooled liquid water (L) transforms to solid ice Ih upon ice nucleation (L → ice Ih + L). While ice nucleation occurs in the ice Ih stability field at −41 °C and 28 MPa the pressure increases instantaneously to 315 MPa into the ice II stability field. At this point, both phases, liquid water and ice Ih are metastable. The coexistence of these two phases was confirmed by Raman spectroscopy and could be traced down to −80 °C. The pressure along this low-temperature metastable extension of the ice Ih melting curve was determined by means of the frequency shift of the ice Ih peak position using both the O–H stretching band around 3100 cm−1 and the lattice translational band around 220 cm−1. At −80 °C and 466 MPa the super-cooled ice Ih melting curve encounters the homogeneous nucleation limit (TH) and the remaining liquid water transformed either to metastable ice IV (ice Ih + L → ice Ih + ice IV) or occasionally to metastable ice III (ice Ih + L → ice Ih + ice III). The nucleation of ice IV resulted in a pressure drop of about 180 MPa. Upon subsequent heating the pressure develops along a slightly negatively sloped ice Ih–ice IV equilibrium line terminating in a triple point at −32.7 °C and 273 MPa, where ice IV melts to liquid water (ice Ih + ice IV → ice Ih + L). Hitherto existing experimental data of the ice IV melting curve (ice IV → L) were found to be in line with the observed ice Ih–ice IV–liquid triple point. If, on the other hand, ice III nucleated at −80 °C (instead of ice IV) the associated pressure drop was about 260 MPa. The ice Ih–ice III–liquid triple point was determined at −22.0 °C and 207 MPa (ice Ih + ice III → ice Ih + L), which is in agreement with previous experimental data.