Shallow deposits of halite are well known to be impermeable to brines and hydrocarbons, yet recent studies of deeply buried salt point toward considerable water-halite interaction. We have resolved this apparent paradox via experimental determinations of the dihedral angle for the (static) halite-H2O system at temperatures of 20 to 300 °C and pressures from atmospheric to 2 kbar. The halite-brine dihedral angle, which controls pore-fluid connectivity and hence permeability, is found to be a sensitive function of both temperature and pressure. In sedimentary basins with normal geothermal gradients, halite bodies at depths exceeding 3 km will contain a stable interconnected brine-filled porosity, resulting in permeabilities comparable to those of sandstones. Our results explain recent observations of major salt-fluid interactions at depth, and suggest that deep-rooted salt diapirs may act as conduits for basinal formation water. They also have important implications for the flow of brines around heat-producing canisters of high-level nuclear waste buried in natural rock salt.