ComputerX
Registered
Assuming:
- There is only one clear path and no chance of getting lost;
- The floor is course sand that won't silt even with a group of divers swimming through the cavern;
- It is so spacious that many divers can pass through at the same time;
- The cavern has multiple large entries and exits; and
- It is well illuminated.
What are the risks of diving in these "caverns"? What could go wrong?
I hope I am not breaking some unwritten scubaboard protocol here, but I will try to answer the poster's question. I'm taking the assumptions as facts.
This is based on things that have actually happened to my girlfriend, who is a much more experienced diver than I am. Two while I was there, and one before I met her.
Before I met her she was swimming through a lava tube that required the divers to go single file. She hit the top of her tank on the overhead and knocked the valve loose. She says she still had air, but she had to swim quite a distance with bubbles everywhere before it could be fixed. She describes breathing getting difficult so she might have just been breathing the air out of the hose. It was fixed underwater and the dive continued. A DIN valve might mitigate the risk. Diving sidemount might help. I think the correct answer is to make sure you have redundant gas for your entire dive, not just portions of it.
In November we dove a cenote. She was hesitant, but the guide assured us it was always wide open and there was no reason to be worried. She told about the previous story and explained that she really didn't like enclosed spaces. She got talked into the dive. We got to an area that was wide but only about five feet from floor to ceiling. She got very buoyant and was pinned to the ceiling. The guide and I were able to keep her down long enough to swim her out of the area. The correct answer is to not let yourself get talked into a dive, but that is really hard to do. Neither of us knew what the inside of the cave looked at. From the descriptions and pictures I thought it would be fine. If I had known what we were in for I would have tried to keep her out. The guide tried to talk her into the second dive also. She wavered, but with a lot of pressure from me, she sat that one out. I think it was a good thing. I love the caverns and I want to progress to caves some day. She likes warm, shallow tropical water. Obviously the guide should not pressure divers. Being a little overweight might be a good idea. If a diver gets nervous and won't breathe all the way out they can have a hard time sinking. a little extra weight will get them down.
Over Christmas we were in Kona. One dive took us into a tiny cavelet (more like a hollow in the rock) and out a "lava tube" that was really a hole. It was, maybe, eighteen inches long. As she was going out, the surge grabbed her and gave her a good spin. That wasn't a big deal. But surge is an issue going through confined areas. Be aware of it. Have three or more professionals. One to lead, one at the tail, and one at the narrow spot to give assurance. And have a plan for what to do if you get half the group through then decide the surge is too strong for the rest of the group.
A common thread here is that the divers didn't know what they were getting into before the dive. Even with my few dives, I have been in that situation. A dive leader takes off into what looks like the mouth of a cave, and I have to decide if I am going to follow, or if I am going to risk getting separated from the group.