Some scientists believe that glaciers may have existed on many of the volcanoes in Tharsis including Olympus Mons, Ascraeus Mons, and Pavonis Mons. Ceraunius Tholus may have even had its glaciers melt to form some temporary lakes in the past. The smoothness and flatness of the Ceraunius Tholus caldera floor suggests that in the past meltwater accumulated in a caldera lake.