I went caving about 15 years ago, and to my surprise we did not use any kind of battery-operated lights when we went into the caves. Instead, we had a flask-like metal tin that unscrewed into two parts. In the top part went about three cups of water. The bottom part we filled with pieces of "carburo" mineral (can't be bothered to look it up; I'm sure someone can give the name). The top bit leaked water very slowly onto the pieces, at which point a reaction took place that formed a gas. This tin hung from our belts, and there was a rubber pipe that connected it to a little open lamp at the front of our hard hats. All you needed to do was take a lighter to the lamp and voila, you had a flame to see by. The gas seemed to last forever, too.
We were told - but maybe this was just the instructors defending their setup - that flashlights could run out of batteries, but that with their carburo system, just by having a lighter you were sure of a light all the time.
