Phase diagram of water under pressure and possible high-pressure-low-temperature treatments (arrows). The phase transition lines are based on the data reported by Bridgman (2). Prolonged transition lines are frequently observed due to metastable states, as in the case indicated for the ice I-to-liquid and ice I-to-ice III phase transitions. Treatment 1 consisted of high-pressure treatment in the liquid state of water at subzero temperatures (cooling during pressurization). Treatment 2 consisted of high-pressure treatment of ice I without phase transition (maximum pressure of about 210 MPa). Treatment 3 consisted of high-pressure treatment of ice including phase transitions between the ice polymorphs (pressure higher than 210 MPa).