Liquid-liquid phase separation in supercooled water

Although H2O has been the topic of considerable research since the beginnin g of the century, its peculiar physical proper ties are still not well unde rstood. We discuss recent experiments and simulations relating to the hypot hesis that, in addition to the known critical point in water (below which t wo fluid phases-a lower-density gas and a higher-density liquid-coexist), t here exists a "second" critical point at low temperatures (below which two liquid phases-a higher-density liquid and a lower-density liquid-can coexis t).