Condensed matter - One substance, two liquids?

Water, like any other liquid, can be cooled below its freezing point without crystallizing. The physical properties of supercooled water are unusual: the lower its temperature, the easier it is to compress, and the more pronounced its anomalous tendency to expand when cooled. As if these characteristics were not peculiar enough, there is now evidence for the coexistence of two different forms of supercooled water. A phase transition between two liquid forms of a pure substance has never been observed before.