An account is given of the exploration of Charterhouse Cave, including early work by this Society, more recent excavations by the Sidcot School Speleological Society and the final opening in 1981. The accessible passages are briefly described. The cave contains a typical Mendip cave fauna, with a well developed twilight community and a troglobite fauna comprising two Crustacea and a springtail. The population of the troglophile, Speolepta leptogaster, will probably be most sensitive to the opening of the system. The cave is developed in a structurally complex monoclinal flexure, with strong jointing perpendicular to the east-west fold axis. Phreatic remnants are present as high as 240 m AOD, but the local water-table elevation declined gradually with time to below the present end of the system. A sequence of six vadose trenches adjusted to temporary still-stands of the local water-table are proposed and evidence of nine sometimes spatially discrete fill phases cited to support this contention. These fills also assist in the recognition of a minimum of five separate significant inlets to the system, which have varied in importance through time. Down-dip modification of the existing mature phreatic system is stressed. This evolution scheme is contrasted with that of Ford (1964) for the nearby G.B. Cave, where it may also apply.