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.