Samples of high-density amorphous water (HDA) were prepared by compression of hexagonal ice (ice Ih) in a piston-cylinder apparatus at 77 K up to 1.8 GPa, recovered at ambient pressure under liquid N2 and characterized by X-ray diffraction and differential scanning calorimetry. Thereafter HDA samples were reloaded into the piston-cylinder apparatus, compressed at 77 K up to 1.8 GPa, recovered under liquid N2 and characterized. Formation of ice XII on compression of HDA was observed only when a sudden pronounced pressure drop occurred at pressures â ³1.1 GPa. Pressure drops below this threshold value, or occurring during the initial compression of ice Ih, did not induce formation of ice XII. Pressureâ displacement curves obtained on compression of ice Ih are consistent with formation of ice XII from HDA, and not from ice Ih. We propose that shock waves generated by the pressure drops cause transient local heating up to the temperature range of the ice V domain, and that this induces nucleation and crystal growth of ice XII. The threshold value of â 1.1 GPa pressure necessary for the HDA â ice XII transition indicates that the density of HDA has to reach a certain minimal value before conversion into ice XII becomes possible. Ice XII could thus be formed .italv;ia HDA by meteorite impact on the surfaces of icy satellites of the outer planets whenever the temperatures are low.