Extremely acidic (pH<1) saline lakes and ground waters existed in the midcontinent of North America during the mid Permian. Acidic saline lake systems throughout southern Australia seem to be the best modern analogs. We compare and contrast the Permian Opeche Shale of North Dakota and Nippewalla Group of Kansas to modern Australian salt lakes such as Lakes Gilmore and Swann in southern Western Australia and Lake Tyrell in northwest Victoria. Both the Permian and modern systems are characterized by: (1) ephemeral saline continental playas hosted by red siliciclastic sediments, (2) evaporite minerals, including abundant sulfates, (3) Al-Fe-Si-rich waters with low pHs, (4) acidophilic microbes, and (5) paucity of carbonates. However, there are some minor variations in pH, evaporite minerals, and water geochemistry. Comparing modern and ancient extremely acidic systems may lead to the development of criteria for the recognition of acidic deposits in the rock record. In addition, it may have implications for the study of paleoclimate and paleoweathering trends, especially on supercontinents, as well as possible links to Martian environments.