Later rock paintings and engravings showing hippos being hunted . They have been found in the mountains of the central Sahara from 4,000 â `` 5,000 years ago near Djanet . Later rock paintings and engravings showing hippos being hunted have been found in the mountains of the central Sahara dated 4,000 â `` 5,000 years ago near Djanet in the Tassili n ` Ajjer Mountains .