  While the water molecule is simple, its condensed phase liquid behavior is so
complex that no consensus description has emerged despite three centuries of
effort. Here we identify features of its behavior that are the most peculiar,
hence suggest ways forward. We examine the properties of water at the
boundaries of common experience, including stable states at high pressure, the
supercooled state at normal and elevated pressure, and the stretched ("negative
pressure") state, out to the limits of mechanical stability. The familiar
anomalies at moderate pressures (viscosity and density (TMD) behavior, etc.),
are not explained by H-bond breaking, according to common bond-breaking
criteria. A comparison of data on the TMD, at both positive and negative
pressures, with the predictions of popular pair potential models, shows
dramatic discrepancies appearing in the stretched liquid domain. This prompts
questions on the second critical point (TC2) hypothesis that has been guiding
much current thinking. We turn to related systems for guidance, reviewing a
hierarchy of water-like anomalies. We conclude that water models are far from
complete and that proper understanding of water will depend on success in
mastering the measurement of liquid behavior in the negative pressure domain -
which we discuss.
