Hexagonal ice (Ih) is a molecular crystal with an unusually open structure, with shafts running perpendicular to the basal plane (0001) surface. As a result, the (0001) face can be penetrated by small molecules at collision energies (1 eV) which are more than an order of magnitude lower than those for which penetration of metallic or covalently bonded crystals, by atoms, becomes possible. This is demonstrated theoretically in calculations on HCl+ice. This prediction can be tested by molecular beam experiments on scattering of HCl from clean and adsorbate covered ice.