The slopes of the crater that contains Lake Nemi also conceal an important work of hydraulic engineering: an artificial effluent that probably dates back to the 5th C. B. C.
This narrow shaft, 1,635 mt long by 80 cm. wide, was bored through the side of the crater wall and comes out into the Ariccia valley.
This system was devised on the one hand to ensure that the level of the lake never exceeded a certain height, in order to protect the lake the shores and the temple of Diana, and on the other so the water flowing out of the side of the mountain could be used to irrigate the valley below.
The effluent then continued in the open as far as Arda and the sea.
Two teams of workers were used to excavate the effluent, each from opposites sides of the rock face, and they then met more or lest half way through.
After centuries of neglect, the effluent was restored in the Thirties to lower the level of the lake and retrieve Caligula’s ships.
Now it can once again be visited, using the ancient service gallery that runs along side it for a short stretch.