The way in which a three-dimensional network of water molecules accommodates an excess proton is hard to discern from the broad vibrational spectra of dilute acids. The sharper bands displayed by cold gas-phase clusters, H+(H2O)n, are therefore useful because they encode the network-dependent speciation of the proton defect and yet are small enough to be accurately treated with electronic structure theory. We identified the previously elusive spectral signature of the proton defect in the three-dimensional cage structure adopted by the particularly stable H+(H2O)21 cluster. Cryogenically cooling the ion and tagging it with loosely bound deuterium (D2) enabled detection of its vibrational spectrum over the 600 to 4000 cm-1 range. The excess charge is consistent with a tricoordinated H3O+ moiety embedded on the surface of a clathrate-like cage.