The Fucine Lake was a large (c. 12 mile long) lake in the Apennines. It was hemmed in by the mountains, filled with runoff from them, and had no natural outlet. The locals had been asking the Roman emperors to drain it at least since Augustus' time so that they could farm the land covered by it. Claudius accepted the challenge, with inducement from some people who offered to pay for the work if he awarded them the land thus reclaimed. 30,000 men dug for 11 years through three miles of mountains, and leveled part of a hill, to create an outlet for the water. It was the longest tunnel in the world until the mid C19. See further engineering here.
References: Suetonius Claudius 20.