  One of water's unsolved puzzles is the question of what determines the lowest
temperature to which it can be cooled before freezing to ice. The supercooled
liquid has been probed experimentally to near the homogeneous nucleation
temperature TH{\approx}232 K, yet the mechanism of ice crystallization -
including the size and structure of critical nuclei - has not yet been
resolved. The heat capacity and compressibility of liquid water anomalously
increase upon moving into the supercooled region according to a power law that
would diverge at Ts{\approx}225 K,(1,2) so there may be a link between water's
thermodynamic anomalies and the crystallization rate of ice. But probing this
link is challenging because fast crystallization prevents experimental studies
of the liquid below TH. And while atomistic studies have captured water
crystallization(3), the computational costs involved have so far prevented an
assessment of the rates and mechanism involved. Here we report coarse-grained
molecular simulations with the mW water model(4) in the supercooled regime
around TH, which reveal that a sharp increase in the fraction of
four-coordinated molecules in supercooled liquid water explains its anomalous
thermodynamics and also controls the rate and mechanism of ice formation. The
simulations reveal that the crystallization rate of water reaches a maximum
around 225 K, below which ice nuclei form faster than liquid water can
equilibrate. This implies a lower limit of metastability of liquid water just
below TH and well above its glass transition temperature Tg{\approx}136 K. By
providing a relationship between the structural transformation in liquid water,
its anomalous thermodynamics and its crystallization rate, this work provides a
microscopic foundation to the experimental finding that the thermodynamics of
water determines the rates of homogeneous nucleation of ice.(5)
