The processes of phase transformation of water during ice melting are considered. Possible structures of liquid water are analyzed, corresponding to the results of experimental measurements and computer simulation. It is shown that when ice melts, the tridymite structure breaks down into individual clusters, the lifetime of which ranges from 2 to 8 ps and increases with an increase in their molecular weight. A sharp increase in the density of liquid water at 00C is explained by the formation of clathrates during the introduction of water molecules into the cavity of the structure and an increase in the degree of coordination from 4.0 to 4.34. The most stable are water clusters, consisting of 12–20 molecules, the mixture of which determines the density of the liquid phase. Molecules in the middle of clusters can form additional hydrogen bonds with their nearest neighbors in the cluster framework. Thus, one or two framework molecules can have three acceptor and two donor bonds of A3D2 type. The central molecule is most likely of type A1D2 or A1D1. An example of calculating the density of water when taking into account clathrates and vacancies is given.