There’s more to water’s phase diagram than the simple solid, liquid, and gas of everyday experience. The low-temperature, high-pressure portion of the diagram, for example, contains a spiderweb of solid–solid phase transitions between various stable crystalline structures. And a 25-year-old theory predicts that deep in the supercooled regime, liquid water, too, undergoes a phase transition between two structurally distinct forms. Experiments have hinted at that liquid–liquid phase transition (see, for example, Physics Today, December 2013, page 16). But it’s never been directly observed.

Now Stockholm University’s Anders Nilsson and his colleagues have produced some of the strongest evidence yet in support of the two-liquid theory.1 The researchers scattered femtosecond x-ray pulses off a stream of water microdroplets that had been evaporatively cooled under vacuum to as low as 227.7 K, or −45.4 °C. From different portions of the scattering curves, they extracted four thermodynamic and correlational quantities....

You do not currently have access to this content.