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Evaporites hosted by red beds (red shales and sandstones), some 275-265 million years old, extend over a large area of the North American mid-continent(1). They were deposited in non-marine saline lakes, pans and mud-flats(2), settings that are typically assumed to have been alkaline. Here we use laser Raman microprobe analyses of fluid inclusions trapped in halites from these Permian deposits to argue for the existence of highly acidic (pH <1) lakes and ground waters. These extremely acidic systems may have extended over an area of 200,000 km(2). Modern analogues of such systems may be natural acid lake and groundwater systems (pH similar to 2-4) in southern Australia(3-9). Both the ancient and modern acid systems are characterized by closed drainage, arid climate, low acid-neutralizing capacity, and the oxidation of minerals such as pyrite to generate acidity. The discovery of widespread ancient acid lake and groundwater systems demands a re-evaluation of reconstructions of surface conditions of the past, and further investigations of the geochemistry and ecology of acid systems in general.