Over the years, there have been a number of cave-diving fatalities in which the victims either knew the way out, or had a good chance of finding the way out. They simply did not have sufficient breathing gas to make it.
- Sport divers, in particular, tend not to think about surfacing until the pressure in their cylinders drops to 35-70 bars/500-1,000 psi. This may seem like a no-brainer, but if divers use 140 bars/2,000 psi or more to penetrate a cave, it is going to be very difficult to exit that cave on 70 bars/1,000 psi or less.
- If you factor in the unforeseen (loss of visibility, the need to share gas with other divers, etc.), the odds of divers surviving under such circumstances are exceptionally slim.
Trained cave divers learn to keep at least two thirds of their starting gas volume in reserve to exit. This Rule of Thirds provides a sufficient safety margin. This is true even in instances where divers’ exits have been slowed by poor visibility or momentary loss of guideline. It is also true when gas consumption has increased due to apprehension or the need to share air with another diver.