Marmore Falls is the tallest man-made waterfall in the world. Its total height is 165 meters and it was built by the ancient Romans. Water runs from the upper reservoir on the Nera river several times a day (monthly schedule).
There's a large park and a tourist center around the waterfall, and also many walking trails. By using the trail number 1 you can get to the top observation deck in 40 minutes, but we didn't go there. You can also come there by car, entrance with the same ticket. The waterfall is illuminated in the evening.
The problem for photographers is water "fog", which appears after the waterfall is turned on. Probably it depends on the temperature and weather.
One of the trails can get you to the Lovers' Tunnel, which ends directly under the waterfall. It's impossible to stay dry there :)
Map of the area (PDF)
Getting there: The nearest railway station is Terni, then take either a bus #7 (schedule, not very clear) or #21 or a taxi, 10-20 minutes to go. Your stop is "Cascata delle Marmore". If you are using GPS, then drive to PIAZZALE FELICE FATATI 1, 05100 COLLESTATTE PIANO (TR) or to the coordinates 42°33'23.90'' N-12°43'09.23'' E. Coordinates of the upper observation deck are VOCABOLO CASCATA 30, 05100 COLLESTATTE PIANO (TR) or 42°32'59 .99''N-12°43'00.87''E.