Ferretas, yes fantastic - except in lightning storms, where you are tied to the worlds largest lightning conductor...
10 or so people a year get fried in this way...
The tunnels themselves date from the first World War, when the Italians were on our side. Many of the summits are riddled with them, and they are worth an explore in their own right. There are lots of artefacts down there in the ones that are off the beaten track... We found old nespapers, bottles, ammunition etc. etc.
There are also a few graves on some of the paths, just a couple of sticks marking them sometimes.
For an underground experience, I'd recommend the Tri Zimmen area.
There are good guide books around, but i don't like the Cicerone ones, as they are often poorly translated. They also use Italian place/peak names, whereas most signs in the mountain areas are in German...