Abstract: Liquid water displays a number of anomalies, including the famous density maximum at 4°C and the compressibility minimum at 46°C. Moreover, the density decreases markedly and the compressibility increases rapidly as the temperature is further decreased below 0°C and the liquid becomes supercooled. Finally, the density decrease upon freezing is also anomalous.

In this talk, I will explore possible microscopic and thermodynamic origins of these anomalies; present experimental, simulation, and theoretical evidence for the presence of a liquid-liquid critical point in deeply supercooled water; and argue that this critical point, even though far removed in temperature and pressure from ambient water, is nonetheless responsible for its anomalies.