Back in the early 70s if you wanted to get to Whittier, Alaska you had to take the Alaska Railroad, a plane or a boat. You could load a car onto the train at Portage and then ride the ~9 miles to Whittier. The train went through two tunnels on the way and at the time they were only big enough for one set of tracks, although now there is also a road so you can get there by just driving.
One weekend a friend of mine and I took the train to Whittier and then hiked back into Bear Valley -- the valley between the two tunnels. In order to camp there we had to walk through the longest tunnel which was about 2 1/4 miles long.
We only had one of the old plastic Mallory 2AA side by side lights (incan) and no spare cells. We talked it over and decided that the prudent thing to do would be to just slowly shuffle through the tunnel in the dark on our way in so we would be sure to have the light if one of us got hurt and the other had to run back through the tunnel for help.
So, we walked through in the dark, and wow, was it dark! For a while the tunnel entrance was just the tinyest possible speck of dim light. It was the darkest place I've ever been.
Also, a train went through while we were in there. We took off our packs and both sat down against the side of the tunnel. I have never been so close to something so loud and so big moving so quickly. That was different.
The Duracell Mallory was a good backpacker's light for it's time but what I would have given for a CMG Infinity with a green or blue/green LED -- and it's 41 hour runtime.