  Despite its fundamental and technological importance, a microscopic
understanding of the crystallization process is still elusive. By computer
simulations of the hard-sphere model we reveal the mechanism by which thermal
fluctuations drive the transition from the supercooled liquid state to the
crystal state. In particular we show that fluctuations in bond orientational
order trigger the nucleation process, contrary to the common belief that the
transition is initiated by density fluctuations. Moreover, the analysis of bond
orientational fluctuations shows that these not only act as seeds of the
nucleation process, but also i) determine the particular polymorph which is to
be nucleated from them and ii) at high density favour the formation of fivefold
structures which can frustrate the formation of crystals. These results can
shed new light on our understanding of the relationship between crystallization
and vitrification.
