Subiaco is more than two thousand feet above the level of Rome, and all the foregoing description applies to the part of the river above Subiaco. In all this upper part of the stream, the river Anio has very much the character of a large mountain-torrent rushing through the rocks with great violence. It usually appears to be a clear stream of beautiful water, excepting in time of floods, when the clayey soil is brought down into the stream and makes it muddy. To guard against this evil, Nero made his great lakes, which do not correspond at all to the usual English idea of a lake, by which is commonly understood a considerable sheet of water, like the Swiss lakes or the lakes of Westmoreland, Scotland, and Wales. The Roman word _lacus_, and the modern Italian _lago_, may mean a lake of this description also; but it includes a reservoir of water of any kind. We are expressly told in the _Breviarium_, at the end of the Catalogue of the Regionaries, that a _lacus_ is a well, _puteus_, and the numerous _lacus_ of that Catalogue are the same as the _castella aquarum_ of Frontinus, within the walls of Rome. These justly-celebrated lakes of Nero are in fact portions of the river Anio, intercepted in a gorge of the rocks about a mile above Subiaco, and are formed by cutting away some large pieces of the rock on each side in large masses, and with these building a great high and massive wall across the stream, forming an effectual barricade or dam to stop the water and raise it to the level of the top of this great wall. There were three of these walls across the stream, over each of which the river fell in tremendous cascades. The first loch (_lacus_) commenced at the Mola di Ienne, where the first great wall of enclosure is situated. The second at the cascade of the river, under the great monastery of S. Benedict, and called “the Sacred Specus,” from a cave in which the saint is said to have lived, extends to the bridge called Ponte di S. Mauro, where the _specus_ of Trajan commenced. The third is from the Ponte di S. Mauro, through a gorge, where the wall of enclosure is visible. Here was the Piscina Limaria of Claudius, and the _specus_ of the Anio Novus originally commenced at the Emissarium, restored by Cardinal Barberini. This was made by Claudius, but was abandoned by Trajan, because the earth from the adjoining fields had fallen between the river and the _specus_.