We demonstrate that even far below the melting point, thermodynamic data on water in porous systems indicate that part of the water is in the liquid state. This gives a possibility to study some fractal properties of the pore structure, to obtain thermodynamic data of a largely undercooled melt and also to obtain estimates of interfacial energies. The latter data indicate that ice in pores is covered by a quasi-liquid layer at temperatures down to 100 K.