All tunnelling (well, construction work for that matter) is potentially dangerous, but when you're working with such powerful and incredibly noisy machinery, 10,000 volt electric supply lines, an unsupported tunnel with a rough rock surface, a foot or more of water on the floor and gallons pouring in all the time...all in partial darkness and restricted space...
I used to call it 'a vision from hell'.
It was always 'interesting' probing the ceiling with long steel bars to locate insecure rock so that it would come down....hopefully in front of you before falling on someone else's head.