In 1927 Professor Waterman of the University of Michigan undertook a special study of Babylonian topography for the purpose of locating the site of Opis (Babylonian Upi, Upa) the older Sumerian Akshak, capital of one of the oldest kingdoms in Mesopotamia. The general location of Opis is well defined by the inscriptions of Nebuchadrezzar at Wadi Brissa and Nahr el-Kelb, who states that for the defence of Babylon he built a wall from Sippar on the Euphrates to ‘ above ’ Opis on the Tigris, a distance of five beru, about twenty miles. Since with the help of this brick-faced earth wall he fashioned great bodies of water ‘ like the sea ’, twenty beru in length (probably in circumference), it is clear that a watercourse formed part of this system of defence. Xenophon mentions Opis as located above the modern Bagdad, but Strabo clearly places it in the district of the later Seleucia, stating2 that the Tigris was navigable as far as Opis and Seleucia.