I am aware that dwindling gas is tha nature of diving. I was referring to a cave-in or a collapse trapping one inside with no way out. Obviously you would make sure to leave a reserve for exiting and a reserve for contingencies. I'm more than likely just being paranoid. Looking into cavern courses and instructors for more information.I'm afraid your gas dwindling is a characteristic of diving. Maybe some more open water experience would build knowledge and confidence. The power of rational thought is vital in all diving but critical in an overhead environment. In a cavern course you can always the external light source and be able to access it.