Liquid water starts to expand about four degrees above the temperature at which it turns to ice — a property that explains why lakes freeze from the top, protecting the animal and plant life beneath. This unusual behaviour (most liquids contract when they are cooled) persists even when water is 'supercooled' below its freezing point.

Simulations suggest, however, that supercooled water should start to contract at low enough temperatures — typically about 70 degrees below its freezing point. To reach these low temperatures without ice crystal nucleation, Sow-Hsin Chen of the Massachusetts Institute of Technology in the USA and colleagues in Taiwan and Canada cooled heavy water (D2O) that was confined to the narrow 1.5 nm channels of mesoporous silica1.

The small volume of the pores allowed Chen's group to cool water more than 100 degrees below freezing. Using a neutron diffraction technique that is sensitive to the mass density of the water in the pores, Chen and colleagues showed that the D2O first expanded on cooling, and then started to contract. The findings suggest that water, which behaves so differently from most liquids, does actually start to act 'normal'.