Ascending from Hadrian’s Villa to the point where the Anio issues from a valley dividing the Æquian from the Sabine Mountains, we find the river winding round a considerable hill, partly clothed with groves of olive, and rising to the height of 830 feet. At the back of this hill the river has forced a passage for itself through the limestone rocks which threaten to impede its exit from the upper valley, and falls in a tremendous cataract down a precipitous cliff of 320 feet in height to the lower level. The water is strongly charged with carbonate of lime, which is constantly being deposited in the shape of masses of travertine in the channels through which the stream runs, especially where the water, in consequence of the violent agitation caused by its rapid descent, parts quickly with the carbonic acid gas contained in it. The course of the stream is from time to time blocked up by its own formations of stone, and it is forced to open new passages for itself. From this cause the city of Tibur, which stands on the hill, close to the point where the river falls to its lower level, has always been subject to violent and dangerous inundations. The great inundation of 1826 proved so formidable that it was at once resolved to divert the course of part of the river and provide it with an artificial outlet. This was effected by boring two tunnels through Monte Catillo on the east of the city, through which any rush of water can be allowed to pass and fall harmlessly into the lower valley. A part of the river water is always allowed to pass through these tunnels, and forms at their lower end a magnificent cascade. Another part passes under the bridge called Ponte S. Gregorio and then rushes through a fantastic grotto of travertine blocks called by the local guides Grotta di Nettuno, and joins the stream from the tunnels at the bottom of the valley. A third portion of the Anio is diverted just above the bridge into canals apparently of very ancient date, which, passing completely through the centre of the town, are used as the motive power of watermills and factories of various kinds, and then fall again into the main stream at various points of the romantic cliffs on the western hill side. These form the wreaths of “snow white foam” so celebrated as the cascades of the Anio, and explain perfectly the expression of Horace: O headlong Anio, O Tiburnian groves, And orchards saturate with shifting streams.