Using shock wave reverberation experiments, water samples were quasi‐isentropically compressed between silica and sapphire plates to peak pressures of 1–5 GPa on nanosecond time scales. Real time optical transmission measurements were used to examine changes in the compressed samples. Although the ice VII phase is thermodynamically favored above 2 GPa, the liquid state was initially preserved and subsequent freezing occurred over hundreds of nanoseconds only for the silica cells. Images detailing the formation and growth of the solid phase were obtained. These results provide unambiguous evidence of bulk water freezing on such short time scales. © 2004 American Institute of Physics