The puzzling behavior of water at very low temperature

Although H2O has been the topic of considerable research since the beginnin g of the century, most of its anomalous physical properties are still not w ell understood. First we discuss some of the anomalies of this "complex flu id.'' Then we describe a qualitative interpretation in terms of percolation concepts. Finally, we discuss recent experiments and simulations relating to the liquid-liquid phase transition hypothesis that, in addition to the k nown critical point in water, there may exist a "second'' critical point at low temperatures. In particular, we discuss very recent measurements at Ts ukuba of the compression-induced melting and decompression-induced melting lines of high-pressure forms of ice. We show how knowledge of these lines e nables one to obtain an approximation for the Gibbs potential G(P,T) and th e equation of state V(P,T) for water, both of which are consistent with the possible continuity of liquid water and the amorphous forms of solid water .