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Until the end of the 20th century, the sinkhole-lakes of Layla and Al-Kharj formed oases in central Saudi Arabia. They were fed by ascending groundwater from the karstified Upper Jurassic anhydrites of the Arab and Hith formations. Morphologic features of a cave near Al-Kharj and hydraulic heads in wells of the Layla region show that the karstification of the anhydrites was hypogene. The karstification led to zones of collapse in the overlying Cretaceous sediments. Because of the exploitation of the underlying aquifer, the sinkhole-lakes dried up, exposing world-wide singular sublacustrine gypsum tufas on their walls. The gypsum tufas and widespread gypsum crusts in the vicinity of the sinkholes reflect the former ascent of the sulphate-enriched groundwater.