5: Use Three Sources of Light
Although vastly improved over early designs, underwater lights remain among the least reliable of all dive equipment. Even for top-of-the-line models, the question is not whether a dive light will or will not fail but, rather, when.
Being caught in the total darkness of an underwater cave, due to light failure, is not fatal in and of itself. Nevertheless, it has been a major contributory factor in several cave-diving fatalities. Loss of sight, due to light failure, contributes substantially to disorientation and a sense of panic. Panicky, disoriented divers often make mistakes that divers who can see do not.
Divers blinded by loss of light will have difficulty finding the continuous guideline that leads to the cave entrance (assuming they were wise enough to use one in the first place). Even when lightless divers are in physical contact with such a guideline, it will most likely take them substantially longer to reach daylight than it would were they able to see. At this point, the question becomes whether each diver has sufficient breathing gas to make it to the exit, when moving at such a slow pace.
Try the following experiment: Time how long it takes to walk from where you are now to the closest exit. Repeat the exercise with your eyes closed. It will almost certainly take longer. Odds are, if you can see, you can make it to the closest exit while holding your breath. With your eyes closed, however, the odds you can do so are substantially slimmer.
Because of the hazard that loss of light presents, we teach cave divers to carry at least three battery powered dive lights — one primary light and two back-ups apiece. Cavern divers, who keep the cave entrance in sight at all times, learn to carry at least one primary and one back-up light each (the sun counts as their third light source). Although almost every cavern or cave diver eventually experiences a dive light failure, the odds that all members of a buddy team will experience a triple light failure are astronomically slim.