Catching water's hidden transition.

Water behaves in unusual ways when it is cooled below its normal freezing point (0°C) without turning into ice, a phenomenon known as supercooling. In this regime, its density decreases after passing through a maximum, the amount of energy required to raise water's temperature (heat capacity) rises sharply, and water becomes markedly more compressible (1). These phenomena hint at a hidden reorganization of water molecules. However, probing these structural transformations has long been difficult because deeply supercooled water (liquid water cooled well below its freezing point without crystallizing) solidifies on microsecond timescales, which is too short for observations using conventional experimental techniques. On page 1387 of this issue, You et al. (2) report an approach to probe the structure of water in a transient liquid phase before it freezes. The study elucidates structural and thermodynamic characteristics, narrowing down the range of a hidden liquid-liquid transition in supercooled water. [ABSTRACT FROM AUTHOR]