Throughout the whole period of Babylon's supremacy the Persian Gulf, so far from being a channel of international commerce, was as great a barrier as any mountain range. Doubtless a certain amount of local coasting traffic was always carried on, and the heavy blocks of diorite which were brought to Babylonia from Magan by the early Akkadian king Narâm-Sin, and at a rather later period by Gudea of Lagash,[6] must have been transported by water rather than over land. Tradition, too, ascribed the conquest of the island of Dilmun, the modern Bahrein, to Sargon of Akkad; but that marked the extreme limit of Babylonian penetration southwards, and the conquest must have been little more than a temporary occupation following a series of raids down the Arabian coast. The fact that two thousand years later Sargon of Assyria, when recording his receipt of tribute from Upêri of Dilmun, should have been so far out in his estimate of its distance from the Babylonian coast-line,[7] is an indication of the continued disuse of the waters of the gulf as a means of communication. On this supposition we may readily understand the difficulties encountered by Sennacherib when transporting his army across the head of the gulf against certain coast-towns of Elam, and the necessity, to which he was put, of building special ships for the purpose. There is evidence that in the Neo-Babylonian period the possibilities of transport by way of the gulf had already begun to attract attention, and Nebuchadnezzar II. is said to have attempted to build harbours in the swamp at the mouths of the delta.[8] But his object must have been confined to encouraging coastal trade, for the sea-route between the Persian Gulf and India was certainly not in use before the fifth century, and in all probability was inaugurated by Alexander. According to Herodotus[9] it had been opened by Darius after the return of the Greek Scylax of Caryanda from his journey to India, undertaken as one of the surveying expeditions on the basis of which Darius founded the assessment of his new satrapies. But, although there is no need to doubt the historical character of that voyage, there is little to suggest that Scylax coasted round, or even entered, the Persian Gulf.[10] Moreover, it is clear that, while Babylon's international trade received a great impetus under the efficient organization of the Persian Empire, it was the overland routes which benefited. The outcrops of rock, or cataracts, which blocked the Tigris for vessels of deeper draft, were not removed until Alexander levelled them; and the problem of Babylon's sea-traffic, to which he devoted the closing months of his life, was undoubtedly one of the factors which, having now come into prominence for the first time, influenced Seleucus in selecting a site on the Tigris for his new capital.[11]