Water ice X is stable in the 120-400 GPa pressure range, as obtained from lattice dynamical calculations performed using the density-functional theory. Below 120 GPa, it is characterized by one unstable flat phonon band, which generates the disordered ice X structure. Above 400 GPa, ice X has an unstable phonon mode in M, which leads to the Pbcm orthorhombic structure obtained in previous molecular-dynamics calculations [M. Benoit, M. Bernasconi, P. Focher, and M. Parrinello, Phys. Rev. Lett. 76, 2934 (1996)10.1103/PhysRevLett.76.2934]. Therefore, based on lattice dynamics, we propose that the high-pressure low-temperature phase-transition sequence in H2O ice is ice VIII-disordered ice X-ordered ice X-ice Pbcm.