Ice has eleven known crystalline phases (Fig. 1), in which the water molecules are linked through hydrogen bonds into tetrahedral frameworks(1). This uncommonly large number of different solid phases attests to the structural versatility of the water molecule, Here we report the identification of a new, twelfth phase of crystalline ice in the pressure range 0.2-0.6 GPa. The topology of this phase is unlike that of any of the known phases, and contains a mixture of five-and seven-membered rings of water molecules. It has a density similar to that of ice IV, which also occurs in this pressure range within the stability region of ice V. Both phases are likely to be metastable with respect to the less-dense ice V. This region of the water phase diagram thus provides a potential model system for experimental and theoretical studies of metastability.