I got my tanks out of storage yesterday. They were due for the five year required "hydro" certification so I took them in to see my trainer. He worked to get me trained in some advanced certifications, including Divemaster, back around 1984.
He says some tanks like mine, manufactured before 1988, have blown up and killed people. He says they do a current flow test to look for cracks now, so mine should be ok if they pass testing. He graciously loans me two tanks while mine are being tested.
We talk about a couple of his customers who killed themselves last month in a cave system just just north of Tampa. The cave is called School Sink or Wayne's World. It's owned by the National Speleological Society-Cave Diving Section.
Before you can dive this cave, you must have completed 100 cave dives after doing advanced cave-diving training. You must also go with someone who has dived this system before. There are other requirements. It's a tough cave. It has a hydrogen sulfide layer where the water is milky and visibility drops to 1 or 2 feet. There are lots of confusing passages where you can kick up a fine silt that reduces visibility to zero for a long time. The cave is connected to the Gulf of Mexico and there are strong currents based on tides which change direction as the water in the system seeks its own level with the changing tides. It's up to 160 feet deep so you may need to be breathing exotic gasses or doing long decompression stops on the way up.
He tells me that this Colonel from the MacDill Strike Command Base hid his car and snuck in at night with a buddy. There is a gated fence around the entrance with a combination lock. Some divers share combinations. They found the buddy with empty tanks at 100 feet.
The Colonel had double tanks. They found these abandoned. One of the tanks was full of air. Apparently the Colonel got confused or panicked and forgot to activate the crossover.
The Colonel reached his decompression tank, which he had stashed at 40 feet. The tank was pure oxygen, which is toxic below 30 feet. My trainer gets a funny look on his face at this point in the story. I wonder what he's really thinking? They find the Colonel at 50 feet and the partly consumed decompression bottle of pure oxygen at 40 feet.
It is true that pure oxygen will speed up the elimination of excess nitrogen from your tissues, but almost anyone with any training would know it gives you seizures when you breathe it at two or more atmospheres of pressure.
He says he stopped cave diving years ago. After the cave dive he always thought. "everything went right...nothing went wrong"...he began to wonder what the point was.
In cave diving you run a line to follow back. You have three working lights and you begin your exit after consuming 1/3 of your air. Ive been in some caverns. In a cavern, you can see a little daylight from the entrance if you turn off your flashlight. Caves are dark.