The Alban lake belongs to the water system of the Tiber, and has most of its outlets on the western side. It has been supposed that a subterranean communication exists between this lake and that of Nemi, but Nibby asserts that this is impossible, as the level of the lake of Nemi is higher than that of the Alban lake. The circumference of this sheet of water is more than six miles, and it is nearly elliptical in shape. The story of the sudden rise of its waters in the sixth year of the siege of Veii is well known, and the response of the Delphic oracle as given in Livy. Cicero gives a distinct account of the drainage of the lake. “We are told, he says, by the Annalists, that during the siege of Veii, when the Alban lake had risen to an unusual height, a Veientine noble fled to Rome as a deserter, and declared that it was written in the books of fate which were kept at Veii, that Veii could not be taken, so long as the lake was overflowing its banks, and that if the lake were tapped, and flowed into the sea by its own channel and stream, it would be fatal to the Roman nation, but that if the water were so discharged as to make it impossible for it to reach the sea, then the Romans would be victorious. In consequence of this our ancestors contrived that admirable plan for drawing off and dispersing the water of the lake.” From this passage it would seem likely that the whole object of the drainage of the lake was to obtain a constant supply of water for the irrigation of the Campagna. In another passage Cicero states his opinion still more clearly, that the work was really undertaken for the benefit of suburban agriculture. “The Veientine prophecy that if the water of the Alban lake rose above its margin and flowed into the sea, Rome would perish; but that if it were checked, Veii would be taken, in consequence of which the Alban water was diverted, was intended to benefit the suburban farms, and not to secure the safety of Rome.” What appears strange, is that it should have been necessary to appeal to a superstitious motive in the case of a people evidently so far advanced in civilization as to be capable of carrying out an engineering work of such difficulty in a single year.[139] [Sidenote: Emissarium of the Alban lake.] The tunnel which still carries off the superfluous water of the lake is cut through solid peperino and occasional masses of still harder basaltic lava. It is more than a mile and a half in length, from seven to ten feet in height, and never less than four feet in breadth. The height of the edge of the lake basin above the level of its water at the part which is pierced by the tunnel is 430 feet. Three vertical shafts are still discoverable, by which a draft of air was created and the rubbish was removed, and one slanting shaft for the entrance and exit of the miners. The rock was cut with a chisel an inch wide, as may be seen from the marks left upon the sides of the tunnel.