Solid water exists in a large variety of crystalline and amorphous structures
Ice forms more stable and metastable crystal structures than any other known material.
Scientific Explanation
Most substances form one or at most a few crystal structures when they freeze. Water is radically different: at least 20 distinct crystalline ice phases (ice I through ice XX and beyond) as well as several amorphous variants have been identified to date — more than for any other known material. This diversity is a direct consequence of the unique geometry of the water molecule and its ability to form hydrogen bonds in various arrangements.
Ordinary ice (ice Ih, hexagonal) forms at ambient pressure near 0 degrees Celsius. Under increasing pressure, molecules rearrange into more compact structures: ice II, III, V, VI, and VII appear at progressively higher pressures. Some of these high-pressure phases are packed so densely that they are heavier than liquid water — the exact opposite of normal ice.
In addition, there are amorphous forms of ice (LDA, HDA, VHDA) that possess no crystal order. These form through extreme cooling or compression and behave more like a frozen glass. The transformations between these amorphous forms provide important clues about water’s hypothetical second critical point.
Step by Step
Normal Ice (Ih)
At ambient pressure, water forms hexagonal ice -- the familiar form from your freezer. Six oxygen atoms arrange in a hexagonal ring, connected by hydrogen bonds.
Adding Pressure
What happens when extreme pressure is applied to the crystal structure? The open hexagonal arrangement begins to compress.
Ice II and Ice III
Under pressure, molecules rearrange into more compact structures. Ice II and Ice III emerge -- packed more densely than ordinary ice. Some high-pressure phases are even heavier than liquid water.
The Phase Diagram
The temperature-pressure phase diagram reveals distinct stability regions: ice Ih, II, III, V, VI, and VII each exist within specific pressure and temperature ranges.
19 Known Forms
Water forms at least 19 distinct crystalline ice phases -- more than any other known substance. Add amorphous forms like LDA, HDA, and VHDA, and the total exceeds 20. This diversity is unique in nature.
Everyday Relevance
In daily life we encounter only ice Ih — the ordinary ice in drinks, glaciers, and skating rinks. But the variety of ice phases matters for planetary science: on the icy moons of Jupiter and Saturn (Europa, Enceladus), high-pressure ice forms likely exist at depth, profoundly influencing the behavior of these celestial bodies. In materials science, ice polymorphism also serves as a model system for understanding phase transitions in other materials.