However, Ćukovac, Gardoš and Kalvarija hills are not natural features. Zemun loess plateau is the former southern shelf of the ancient, now dried, Pannonian Sea. Modern area of Zemun's Donji Grad was regularly flooded by the Danube and the water would carve canals through the loess. Citizens would then build pathways along those canals and so created the passages, carving the hills out of the plateau. Today it appears that Zemun is built on several hills, with passages between them turned into modern streets, but the hills are actually manmade.