The lattice of magnetic flux lines that can permeate a type ii superconductor, such as the high-transition-temperature copper oxide materials, melts from a solid-like stale to a liquid-like state at a temperature below the superconducting transition temperature. Contrary to the predictions of mean-field theory, this phase transition in Bi2Sr2CaCu2O8 is found to be first-order. The vortex liquid discontinuously expands on freezing.