Whereas the North Coast Tertiary Limestones of Puerto Rico are classic karst locales, their southern counterparts are almost devoid of karst development. The El Convento Cave-Spring System is the most prominent feature of the only large scale karst area developed on the South Coast Tertiary limestones. The karst topography is localized on the middle Juana Diaz Formation, which is a reef facies limestone, apparently because of the high density and low permeability of this zone as compared to the surrounding chalks and marls. In the El Convento System a sinking ephemeral stream combines with the flow from two perennial springs inside the cave. The surface drainage has been pirated from the Rio Tallaboa to the east into El Convento’s subterranean course. The climate is generally semi-arid with 125-150 cm of rain falling principally as short, intense showers during Sept., Oct., and Nov. Sinking flood waters are absorbed by a small sinkhole and appear two to three hours later in the cave. In the dry season this input is absent. The two springs within the cave have a combined inflow to the system of 1.0 m3/min at low flow but half of this leaks back to the groundwater before it reaches the resurgence. The spring waters are saturated with CaCO3 and high in CO2 (26.4 ppm). As the water flows through the open cave it first becomes supersaturated by losing CO2 and then trends back toward saturation by precipitating CaCO3.