Actually, the tunnel was discovered by German researchers, the Romans built it, if I got it right.
Near my place the Romans dried up a huge mountain lake , simply digging a discharge hole thru a mountain and channeling the waters into a stream running some miles away.
Just like a bathtub.
The tunnel, due to lack of maintenance in the middle ages, caved in, the lake filled up again, until it was dried up again in the '800s.
It took 18 centuries to us italians to regain the Romans' skills at civil and hydraulic engineering.