The town of Tivoli, the ancient Tibur, lies 30km/19mi east of Rome in the Sabine Hills, magnificently situated on a limestone ridge extending south from Monte Gennaro (1,271m/4,194ft; cableway), above the ravine carved by the River Aniene. In Roman Imperial times it was a favorite resort of the great Roman nobles, including Maecenas and the Emperor Augustus himself.
To the east of Piazza Rivarola the Ponte Gregoriana spans the gorge of the Aniene, and beyond the bridge is the main entrance to the park of the Villa Gregoriana. The waters of this river are diverted through the Traforo Gregoriano, a double tunnel (270m/297yd and 300m/330yd long) driven through the west side of Monte Catillo in 1826-35 to prevent the floods which had repeatedly devastated the town. The water emerging from the tunnel forms magnificent waterfalls with a total drop of 160m/528ft (volume reduced at night to supply a power station). At the end of the tunnels is the Grande Cascata (108m/356ft), of which there are fine views from the upper and middle terraces. Also in the park are the Sirens' Grotto and, at the end of a gallery, the Grotto of Neptune, through which the main channel of the Aniene formerly flowed. From the entrance to the gallery a path zigzags up to the exit near the two temples.