Although H_2O has been the topic of considerable research since the beginning of the century, its peculiar physical properties are still not wellunderstood. We discuss recent experiments and simulations relating to the hypothesis that, in addition to the known critical point in water (below which two fluid phases-a lower-density gas and a higher-density liquid-coexist), there exists a "second" critical point at low temperatures (below which two liquid phases-a higher-density liquid and a lower-density liquid-can coexist).
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