  The properties of some forms of water ice reserve still intriguing surprises.
Besides the several stable or metastable phases of pure ice, solid mixtures of
water with gases are precursors of other ices, since in some cases they may be
emptied, leaving a metastable hydrogen bound water structure. We present here
the first characterization of a new form of ice, obtained from the crystalline
solid compound of water and molecular hydrogen called C$_0$ filled ice. By
means of Raman spectroscopy, we measure the hydrogen release at different
temperatures, and succeed in rapidly removing all the hydrogen molecules,
obtaining a new form of ice (ice XVII). Its structure is determined by means of
neutron diffraction measurements. Of paramount interest is that the emptied
crystal can adsorb again hydrogen and release it repeatedly, showing a
temperature dependent hysteresis.
