  When water is confined to nanocavities, its quantum-mechanical behavior can
be revealed by terahertz spectroscopy. We place H2O molecules in the nanopores
of a beryl crystal-lattice and observe a rich and highly anisotropic set of
absorption lines in the terahertz spectral range. Two bands can be identified,
which originate from translational and librational motions of the water
molecule isolated within the cage; they correspond to the analogous broad bands
in liquid water and ice. In the present case of well-defined and highly
symmetric nanocavities, the observed fine structure can be explained by
macroscopic tunneling of the H2O molecules within a six-fold potential caused
by the interaction of the molecule with the cavity walls.
