Water normally exists in hydrogen-bonded environments, but a single molecule of H2O without any hydrogen bonds can be completely isolated within the confined subnano space inside fullerene C-60. We isolated bulk quantities of such a molecule by first synthesizing an open-cage C-60 derivative whose opening can be enlarged in situ at 120 degrees C that quantitatively encapsulated one water molecule under the high-pressure conditions. The relatively simple method was developed to close the cage and encapsulate water. The structure of H2O@C-60 was determined by single-crystal x-ray analysis, along with its physical and spectroscopic properties.