The pressure-induced transition of H₂O into the ice X phase, characterized by symmetric hydrogen bonding, is studied using ab initio molecular dynamics combined with ultrasoft pseudopotentials. A good description of the hydrogen bond is obtained only after gradient corrections to the local-density approximation are included. The transition into ice X is predicted at 49 GPa, in good agreement with experiment, when proton quantum fluctuations are treated within mean-field theory. Molecular-dynamics simulations show that a mode-softening description of the transition is appropriate.