Cave diving has one golden rule: never let go of the underwater guideline installed in most submerged caves. The reason why was illustrated on 29 September 1984 when Peter Verhulsel, equipped with a just a single aqualung and a single light, made his first dive into the Sterkfontein Caves, a vast complex of passages and pools in the Transvaal, near Johannesburg.
Verhulsel, the least experienced member of the party, brought up the rear as he and two friends threaded their way through the complex. Twice he let go of the line to explore passages, twice his friends retrieved him. But the third time he relinquished the line, he disappeared.
After six weeks, the search operation was called off. Cavers continued a "dry" search and discovered a hole close to the main cave, through which they uncovered extensive passages, containing footprints. Six weeks to the day after Verhulsel had gone missing his body was found, in complete darkness, next to a sump pool. The post-mortem results showed he had survived for three weeks. "The biggest myth is that cave divers are superheroes. We're all human, and we all make mistakes." Martyn Farr, a cave diver for over 30 years, is talking to a couple of his students in the car park of the Dan yr Ogof National Showcaves Centre in Wales on an overcast afternoon. Early in the morning the divers had lugged their equipment deep into the caves. Geoff Ballard and Dave Thomas are making their first-ever dives at Dan yr Ogof, and mistakes are not on the agenda.
Geoff, the manager of an IT recruitment company, and Dave, a 29-year old lawyer, are newcomers to an activity pioneered in Britain in the 1930s. Since then, it has spread worldwide, from the murky caves of the Mendips to the clear springs of Florida and the cenotes (natural wells) of Mexico's Yucatan peninsula. Cave divers in the United States and Mexico enjoy spectacular conditions: breathtaking visibility, wide, open chambers, warm water and science-fiction scenery. They primarily come from a sport-diving background, and their equipment has evolved to embrace propulsion units and space-age, purpose-built helmets.
But British cave-diving is quite different. Here, divers contend with freezing water and tight, jagged cracks and passages. Preparation has to be meticulous. While British cave-divers can happily tolerate the conditions in Florida and Mexico, divers from the Americas struggle to cope with the standard British sump.
British cave-divers are a small community – Britain's oldest diving club, the Cave Diving Group, founded in 1946, has around 200 members – and, as in all small communities, everyone seems to know everyone else's business. According to Martyn, there is a tremendous camaraderie, and "friendly competition" to find new caves.
On the Dan yr Ogof cave floor, equipment, most of it customised (British cave-diving is a very DIY culture), is laid out with an almost ceremonial care. Air cylinders, regulators, fins, harness, helmet, lights and battery are lined up in a row. The divers prepare in silence, each at their own pace, using talcum powder to squeeze into dry suits. Nothing is rushed. Compasses, watches, knives and reels of line are strapped to forearms. Every item is double-checked and every item is backed up by another, and perhaps a third. It is a process built on redundancy: if a light fails, switch on a second. The chances that all will fail are remote.
We're beyond the public caves now, and the only light sources are the helmet lights. Seven-litre compressed-air cylinders are attached under arms rather than on backs to prevent valves being knocked. There is floodwater from the previous day, so the water is milky and visibility down to less than one foot. For this reason Martyn decides to take the beginners through their first short passage one by one. Geoff volunteers to go first. They splash up to where the water meets the cave wall, then slide under. Ten minutes later, they emerge around the corner.
I ask Geoff and Dave how they felt before their first cave-dives, and the subject they both sidestep is fear. Geoff says that he was frustrated by the bulk of the equipment, Dave that "the water felt quite nice". As Jason Gibb, an ex-British Sub-Aqua Club instructor also taught by Martyn, says: "Your first line of defence is training, your second line of defence is to have ice running through your veins." During a cave dive there is a constant level of stress, and it has to be tackled immediately or you fall into what is called "an incident pit". One problem quickly becomes two, then three. Every diver has moments when his or her breathing rate shoots up but, as Jason puts it: "If you panic, you're screwed".
Martyn has had close shaves himself. On only his sixth dive, he lost his regulator. Incredibly, he breathed water for two to three minutes before making it out. "I felt so humiliated in front of my two companions," he says. "Neither of them ever dived again. After that I knew what drowning was."
Martyn says he was a very different diver in his twenties, but reached a turning point several years ago and decided to become an instructor. "I put in total commitment because I have lost a number of close friends, who were just half a breath away from safety, and if I can save someone that grief and that guilt, I am succeeding." It fell to Martyn to explain to the bereaved partners of those friends he lost what had gone wrong on the dives.
All agree, however, that the end justifies the difficult means. "It is an amazing privilege to be the first to set foot in a new cave," says Martyn, "and must feel exactly the same as being the first to conquer a mountain peak. You've overcome barriers that have held up explorers for years, but the jubilation is immediately tempered by the realisation that it is meaningless if you can't get back."
Martyn started cave diving in 1970, having realised that the quickest way to discover new caves and underground systems was to go through flooded passages. "I never wanted to be a cave diver, I just wanted to be an explorer," he says. "The thrill is that fewer people have set foot in some caves than have walked on the moon, and there are more out there, somewhere."
The National Caving Association (www.nca.org.uk) organises training courses and co-ordinates member clubs all over the country.
The Cave Diving Group train and support British cave divers. They do not recruit members and insist that candidates must be experienced and responsible cavers: www.cavedivinggroup.org.uk.
Most sites are found in limestone caves, and most are on private land. Get permission from the landowner.
Cave-diving principles are completely different from open-water diving practices: there is no "buddy" system so there is no one else to worry about.
The National Showcaves Centre (01639 730 801; www.dan-yr-ogof-showcaves.co.uk) north of Swansea is open seven days a week. Adults £7.50, children £4.50.
Martin Farr's weekend introductory cave-diving course costs £310: www.farrworld.co.uk. Read "The Darkness Beckons", Farr's history of cave diving (Baton Wicks, £25)
Learn the language
Sump: a water-filled cave passage
Snoopy loops: elastic bands made from car inner tubes
Incident pit: where mistakes multiply under stress
Burn time: length of time until a light goes out
The line: a thin rope laid by divers as a route marker
Hypoxia: shortage of oxygen
Call a dive: turn back at any time for any reason
Dry suit: one-piece waterproof diving outfit
The viz: visibility, which can vary wildly
Thirds Rule: divers start return after using one third of air
Do and don't
Double or triple up your equipment and maintain it obsessively
Learn from other's mistakes analyse accidents and resolve not to repeat fatal errors
Get some proper training from a reputable instructor
Sort out any problems as soon as they arise
Share experiences, information and tips in the pub afterwards
Take out an insurance policy
Panic, or you will use up air supply at faster rate
Lose the line or you will quickly become disorientated
Rush your experience. It takes years to develop cave-diving skills
Have a gung ho attitude there's no room for machismo in an underwater cave
Dive if you are hydrophobic, claustrophobic, or frightened of the dark
Trespass on private property