Ice III is a form of solid matter which consists of tetragonal crystalline ice, formed by cooling water down to 250 K at 300 MPa. It is the least dense of the high-pressure water phases, with a density of 1160 kg/m3 (at 350 MPa). The proton-ordered form of ice III is ice IX.
Phase diagram of water, showing the region where ice III is stable.
Ordinary water ice is known as ice Ih, (in the Bridgman nomenclature). Different types of ice, from ice II to ice XVIII, have been created in the laboratory at different temperatures and pressures.