The single-crystal elastic properties of a hydrous silicate garnet, hibschite (Ca3Al2(SiO4)1.72(H4O4)1.28), were measured using Brillouin spectroscopic techniques. The adiabatic bulk modulus of hibschite, KS=99.8¿1.0 GPa, and the shear modulus, μ=64.3¿0.5 GPa, are 40% lower than the bulk and shear moduli for anhydrous grossular garnet Ca3Al2(SiO4)3. This increased compressibility of hydrogarnet is attributed to increased hydrogen bonding with pressure in the H4O4 tetrahedron. Density considerations indicate that hydrogarnet is likely to be stable relative to an assemblage with H2O as a separate phase throughout the upper mantle and probably the transition zone. Assuming garnet to be the sole repository for mantle water, the seismic wave velocities of a ''wet'' eclogitic layer are 6--8% lower than those of dry eclogite. A hydrated eclogitic layer several times thicker than the oceanic crust would probably be required for a water-rich region of the mantle to be seismologically detectable. Lesser quantities of mantle water than those implied by the above scenario may be invisible to seismic techniques. ¿ American Geophysical Union 1993