section_path: Properties/States/Phases of ice and water
section_name: Phases of ice and water
paragraph_text: Phases of ice and water
  
  The normal form of ice on the surface of Earth is ice I, a phase that forms crystals with hexagonal symmetry. Another with cubic crystalline symmetry, ice I, can occur in the upper atmosphere. As the pressure increases, ice forms other crystal structures. As of 2024, twenty have been experimentally confirmed and several more are predicted theoretically. The eighteenth form of ice, ice XVIII, a face-centred-cubic, superionic ice phase, was discovered when a droplet of water was subject to a shock wave that raised the water's pressure to millions of atmospheres and its temperature to thousands of degrees, resulting in a structure of rigid oxygen atoms in which hydrogen atoms flowed freely. When sandwiched between layers of graphene, ice forms a square lattice.
  
  The details of the chemical nature of liquid water are not well understood; some theories suggest that its unusual behavior is due to the existence of two liquid states.