Ice Ih (pronounced: ice one h, also known as ice-phase-one) is the hexagonal crystal form of ordinary ice, or frozen water.[1] Virtually all ice in the biosphere is ice Ih, with the exception only of a small amount of ice Ic that is occasionally present in the upper atmosphere. Ice Ih exhibits many peculiar properties that are relevant to the existence of life and regulation of global climate.[2]
The crystal structure is characterized by the oxygen atoms forming hexagonal symmetry with near tetrahedral bonding angles. Ice Ih is stable down to −268 °C (5 K; −450 °F), as evidenced by x-ray diffraction [3] and extremely high resolution thermal expansion measurements [4]. Ice Ih is also stable under applied pressures of up to about 210 megapascals (2,100 atm) where it transitions into ice III or Ice II.[5]

https://en.wikipedia.org/wiki/Ice_Ih