If ice is stretched , it may form (meta-) stable ultra-low-density ices. Such ices may be stable at ambient pressure (positive) in the presence of interstitial molecules that prevent structural collapse. However under negative pressure (metastable conditions with respect to mixed gaseous water/hexagonal ice), the need for these stabilizing molecules may not be required. At present there are three such ices, two (ice XVi and ice XVII) have been experimentally proven whilst the other (empty clathrate s-III), although strongly supported by modeling, has not yet been experimentally proven.
