Ice-9, in Vonnegut's Cat's Cradle, is a fictional polymorph of water that stays solid at room temperature and converts every drop it touches into more of itself. Real ice doesn't work that way, but the underlying idea — that water takes radically different crystalline forms — is legitimate science. Researchers have cataloged more than 20 distinct phases since 1900. Two more were confirmed this year, and they're the most structurally complex ever found.
The discovery started with a seven-year-old mystery. In 2018, researchers at KRISS, the Korea Research Institute of Standards and Science, were squeezing water between two diamond tips to study pressure-driven phase transitions. For a few milliseconds, the water appeared to pass through an unidentifiable intermediate state before settling into its next form. They didn't have the tools to investigate it.
In 2025, they brought the problem to the European X-Ray Free-Electron Laser in Germany, a machine that fires X-ray bursts bright enough to image crystal structures in detail. When the beam hit the mystery phase, it scattered in about 15 directions. The molecular arrangement that would produce that scattering pattern had a repeating unit of 152 molecules. The phase was designated ice XXI.
A team at the University of Tokyo reproduced ice XXI using different techniques and spotted a neighboring phase, now called ice XXII, with a repeating pattern of 304 molecules.
Both discoveries support Ostwald's step rule, a 19th-century theory that matter under pressure moves through intermediate phases before reaching its most stable state. The practical stakes, according to Quanta Magazine: in pharmaceutical manufacturing, drug crystals can unexpectedly shift phases and ruin entire production batches.
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