A mile and a half from the coast, we stride off the boat and into the sea.
Resetting our masks, we blow powerfully through the breathing regulators on our scuba gear to eliminate residual water. Then, pressing a button to release air with a hiss from our buoyancy jackets, we start to sink.
If you’ve never done it before, there’s nothing to compare with penetrating the dark, reflective surface of the ocean and finding yourself able to see, clearly, for more than 100 feet in any direction.
Similarly, little can prepare you for breathing underwater. It’s miraculous – goes against your deepest instincts – but also faintly sinister, because sucking air through a scuba diver’s regulator makes anybody sound like Darth Vader.
Ninety feet below us extends the seabed. Just getting down there will make this my deepest ever dive – but that’s only the start. Today, I am diving far beyond the level considered safe for recreational divers – and even beyond the threshold for professionals – into a zone where the ordinary gases in our compressed air tank, nitrogen and oxygen, become narcotic and possibly lethal.
Not far ahead of us extends our destination: the Tongue of the Ocean, an undersea canyon 5,000 feet deep.
Descending gradually along the slanted anchor-line towards the clifftop, we gaze into the murky depths beyond. Every few yards, on the descent, the increased density of the water begins to affect my ears. So I go through the routine: squeezing my nostrils and blowing, tilting my head from side to side to equalise the pressures inside and outside.
Having reached 90 feet, we kick towards the edge and hover above the sheer wall of coral, staring into the shady void. And at a sign from our divemaster, Tom, we launch ourselves off the top.
At Small Hope Bay, on the island of Andros in the Bahamas, you can dive in accordance with the regulations of the diving authorities. There’s spectacular coral, and every kind of speciality dive: shipwrecks, “blue holes” (freshwater rivers which run beneath the sea-bed to wash out in mid-ocean), swimming with huge sharks (while they feed) and diving at night (the perfect preparation for a spacewalk).
In my week at Small Hope I have tried all of these.
But in addition to the regulated dives, Small Hope allows divers to take risks. Under a programme specifically exempted from the supervision of the authorities, even unqualified first-timers – like me – can sample dives that old hands would consider perilous.
At Small Hope, divers are taken out to sea as soon as they have signed a disclaimer form and demonstrated, underwater in a hotel pool, the ability to remove and calmly replace the mask and regulator. Simple? Not necessarily, because when you’re deprived, all at once, of the ability to see and to breathe, it’s easy to panic. And panic is a killer. If one diver panics, the others soon follow.
Before you know it, everybody is flailing about – pulling off masks and yanking at regulators – expending energy and air till suddenly there’s none left.
Additionally, before starting, I learned some basic physics, and biology. At 10m below the surface, air occupies just half its ordinary, surface-level volume. It’s important to understand this because if you fill your lungs at 10m and rise to the top without breathing out, your lungs will expand, then burst.
What’s more, a too-quick assent causes gases dissolved in body tissues to come out of solution and form bubbles (a bit like opening a bottle of fizzy water).
Depending on where they appear in the body, these bubbles can cause paralysis, convulsions, numbness, nausea, choking, itching, speech defects, personality changes and an inability to straighten the joints (that’s why decompression sickness is also known as the bends).
What about the descent? That’s hazardous too. The deeper you go, the greater the water pressure. The greater the pressure, the more air it takes to fill your lungs. And one of the principal arguments against going too deep is that the ordinary gases in air, nitrogen and oxygen, become dangerous when taken in excess.
Nitrogen, which constitutes 78 per cent of ordinary air, can cause narcosis at 60 feet below the surface. At 100 feet, the narcosis is unavoidable. Descending below 100 feet seriously impairs decision-making and motor-function: that is, it makes you do something silly, and prevents you doing it properly.
Divers call this the Martini rule: dropping from one zone to the next, they say, has an intoxicating effect equivalent to drinking a Martini. (But this particular form of inebriation is location-specific: soon after you rise out of the dangerous depths, the symptoms disappear.)
There are warning signs – a metallic taste to the air you breathe, numbness around the lips, dizziness – but the symptoms also include a false sense of security and euphoria, so divers who are “narced” may be inclined to disbelieve the signs, even scoffing at the instruments which tell them they’re running out of air.
Narced divers commonly misunderstand hand-signals, literally confusing a thumbs-up (indicating ascent) with a thumbs-down, refusing to act accordingly and having to be dragged back up to their senses. Some have been observed ditching their apparatus in the belief that they can breathe without it; others generously attempt to share their apparatus with fish.
To avoid narcosis, commercial and naval divers use special mixtures of gases, with the nitrogen content reduced from 78 to 68 per cent; or using helium instead.
Breathing pure oxygen is not the solution, because even at relatively modest depths pure oxygen will induce convulsions, leading inevitably to death by drowning. And even ordinary compressed air (which contains only 21 per cent oxygen) becomes a major concern at great depths.
For these reasons, among others, the British Sub Aqua Club recommends 114 feet (35m) as the limit for recreational divers, and 164ft (50m) for dive leaders. The danger threshold for convulsions caused by excessive intake of oxygen is 172ft. But at Small Hope, when we reach 172ft, we, er… keep dropping.
The descent seems to have lasted hours. One of my companions, Rob, drops vertically, with his legs together and arms pinned to his side. Anabel, with arms outstretched, resembles a sky-diver. Me, I try both, then briefly invert myself to drop head-first.
Eventually, at 183ft, we level off where a narrow ledge juts from the face of the cliff. We don’t realise it, but already the nitrogen has exerted its narcotic influence.
For no particular reason I put my legs into a yoga position, inducing clouds of laughter from my companions. (It’s not easy to laugh when you’re diving: your mouth is full of rubber, and snorting through the nose causes a blinding rush of water into your mask.)
Having briefly recovered from this mirth, we decide to carry out an ad hoc experiment in the science of spectroscopy. At this depth, every colour except blue has been filtered from daylight by the sea.
But Rob has brought a tiny torch with him.
Pointing it at the cliff face, he switches it on – and by means of this electromagnetic radiation demonstrates that a chunk of blue coral is actually crimson. (More insane laughter.)
Next, Anabel picks up a lobster from the ledge and hurls it towards me. But in water this dense exceptional strength is required to propel even streamlined missiles, and the lobster tumbles in slow-motion to Anabel’s feet.
Rob retrieves it, and with extraordinary fearlessness applies its claws to his crotch. (Only later do I discover the lobster is not alive.)
Hilarious? Yes: I laugh so heartily at Rob’s lobster-slapstick that for quite a while I can scarcely see through the bubbles. This could possibly be the funniest five minutes of my life. And in fact it seems longer: at a guess, I’d say we’ve been here for hours. But the air, at this depth, does not last long, and Tom soon gives the signal to begin our ascent.
Back at 90 feet, where some of the colour has seeped back into our surroundings, we swim behind Tom for a short while to get the nitrogen out of our bloodstream. Every so often, he turns to point out notable coral – yellow lumps like brains, massive leaf shapes, purple champagne flutes – and we stare with the strenuous gravity of drunkards on best behaviour.
I also chase a ray, and a beady-eyed turtle flutters away from me like some fat bird. Then the ascent continues, with the customary halt at 15ft to prevent the bends.
Back on the boat, I lie on the deck staring at clouds, muttering to myself and sniggering quietly.