You might want to read up on the Battles of the Isonzo, which was the Southern front of the first World War in the middle of the mountains between the Italians and Austro-Hungarians and was one of the two major Austrian lines.

The Isonzo was a tunnel nightmare. The Italians would tunnel towards the enemy using hand drills or even hammer and chisel to minimise noise levels in an attempt to get under key enemy positions, fill them with explosives and blow the whole thing up. The Austrians in turn would dig in the other direction using the same methods. Every so often one side'd stop and listen if they could hear digging noises. If they did it meant the enemy was working as well so they suddenly had to turn the tunnel in the direction of the noises in the hopes of catching their tunnels and blowing them up by planting explosives in their own tunnels before the enemy could finish.

And endless digging battle in the cold, wet and dark rocks in a height of over 2000m at times.

There were several hotbeds of that mine war in the Alps, the most famous probably the Lagazuoi, which has countless kilometers of tunnels from both sides all over and still several talus sites that originated from detonations of tens and hundrets of tons worth of explosive depots deep in the rocks. Or the Italian detonation of the Col di Lana summit.