Cave diving

Is cave diving safe?

No, not in the way most sports on our list are. Even for qualified, experienced divers, a cave dive carries ten or more times the risk of an open-water dive, and for divers without training it is worse than an ascent of a glaciated peak such as Rainier. The best per-dive figures come from British cave diving, where the Cave Diving Group counted dives and deaths over 26 years. For 1981–2005 it estimated 1 death in 4,063 dives across all divers, about 250 micromortsMicromortA one-in-a-million chance of death. It lets you compare the risk of very different activities on one scale.More in the glossary per dive. Qualified divers came out at 1 in 14,219 to 1 in 28,438 dives, or 35 to 70 micromorts. Unqualified divers: 1 in 1,125.

Those numbers rest on nine deaths and British sump diving, which is colder, tighter and murkier than the clear springs of Florida or Mexico. They are rough. But they agree with what every cave diving death review says: training and experience are the biggest differences between who survives and who does not.

For comparison, open-water scuba diving carries about 2 to 5 micromorts per dive.

How cave divers die

The ceiling changes everything. In open water, a diver with a problem can go up. In a cave, the only way out is back along the route in, sometimes hundreds of metres, sometimes through restrictions, often in silt.

Potts, Buzzacott and Denoble reviewed DAN's US records from 1985 to 2015 and traced the causal chain behind each death of a trained cave diver. Drowning was the cause in 41 of 67 cases. In 28 of those, the diver had run out of gas first, and the most common reason was getting lost:

• Silt-outs. Fine silt stirred up by a fin kick or exhaust bubbles can take visibility to zero in seconds. Six of the lost divers lost their way this way.
• Wrong turns in complex cave systems (three).
• No continuous guideline back to open water (two).

Two divers even swam against the line arrows pointing to the nearest exit while their buddies followed the arrows out.

Gas mixtures are the other killer. Seven trained divers had seizures at depth, at least four after breathing from the wrong cylinder, with too much oxygen for the depth. Rebreather deaths rose from one in the first 15 years to seven in the second.

Trained versus untrained divers

The story of cave diving safety is partly a success. The NSS Cave Diving Section records an all-time high of 26 US deaths in 1974. After Sheck Exley's Basic Cave Diving: Blueprint for Survival in 1979 set out rules drawn from accident analysis, and cave training spread, deaths fell to under five a year, and DAN data show fewer than three a year by 2015.

Most of the early dead were open-water divers who swam into caves without training. In 1985–2000, only 31% of US cave diving deaths were trained cave divers. In 2000–2015, it was 62%. That is partly good news, as untrained divers are kept out by signs, access rules and education, but it means the people dying now are often the ones who know the rules.

Of the 67 trained divers who died, 61 died in Florida, home to most US flooded caves. The Devil's system led with 14 deaths.

The rules, and their limits

Cave training is built on a few rules drawn from accident analysis:

1. Thirds. Use a third of your gas going in, a third coming out and keep a third for emergencies. This was the rule broken most often by trained divers who died (20 of 67), and judged relevant in 11 deaths.
2. A continuous guideline from open water to wherever you are.
3. Depth limits for the gas you breathe, with cylinders clearly labelled and checked.
4. Training for the level of cave you enter.
5. Three lights, each lasting the whole dive. In the US review, this rule was never relevant to a death.

The rules work, but they do not cover everything. In the US review, 27 of 67 trained divers who died had broken at least one rule. The rest died following them, from silt, wrong turns, equipment failures and medical events. That is why controllability scores 62, not higher.

Who should not cave dive

• Open-water divers without cave training. Stop at the sign or the end of the daylight. Cavern tours with a guide are a different, much lower-risk activity.
• Divers who are rusty, even with a cave card. Skills such as zero-visibility line following and gas sharing fade.
• Anyone with a patent foramen ovale or past decompression illness, who should talk to a diving doctor before long, deep cave dives.

How we scored it

Fatality risk (40) reflects about 35 to 70 micromortsMicromortA one-in-a-million chance of death. It lets you compare the risk of very different activities on one scale.More in the glossary per dive for qualified divers and far more for untrained ones. Injury risk (55) is moderate because most problems end in death or a clean exit. The environment (15) is among the worst we score: no direct way up and slow rescue. For a sport where the risk sits mostly in judgement, compare free diving and mountaineering.