  Liquids generally become more ordered upon cooling. However, it has been a
long-standing debate on whether such structural ordering in liquid water takes
place continuously or discontinuosly: continuum vs. mixture models. Here, by
computer simulations of three popular water models and analysis of recent
scattering experiment data, we show that, in the structure factor of water,
there are two overlapped peaks hidden in the apparent "first diffraction peak",
one of which corresponds to the neighboring O-O distance as in ordinary liquids
and the other to the longest periodicity of density waves in a tetrahedral
structure. This unambiguously proves the coexistence of two local structural
motifs. Our findings not only provide key clues to settle long-standing
controversy on the water structure but also allow experimental access to the
degree and range of structural ordering in liquid water.
