THE ROOF DIPS IN A BROAD SWEEP and the walls recede; the boulder floor slopes less steeply now. We are at 20m depth, and the vista opens dramatically.
In water as clear as the air in the dry cave above, I can see an unbelievable distance. The rock is a creamy grey, and my appreciation of the immensity of the place is reinforced by the sight of the divers spread out in front, in accordance with our dive-plan.
Slightly ahead, Chris Edwards swims some 10m to my left; Paul Axton cruises perhaps 20m diagonally to my right.
Steve Marsh heads up the arrow formation 30-40m down the line, roughly mid-tunnel. The guys are instructed to stay at least 5m off any rock surface to avoid burn-out from the flash-guns. As shot after shot is fired from the camera, I pray that just one of the frames will synchronise all four flashes.
In the esoteric world of cave-diving, the Nullarbor Plain of Western Australia is fabled for its long and committing dives in awesomely large tunnels. But few people visit, and for good reason.
Remote is the best way to describe this vast area of limestone beside the Great Australian Bight. The Nullarbor is a semi-desert area a gruesome two-day drive from Melbourne, more than 1250 miles east, or a very long days motoring from Perth in the west.
Given the isolation and lack of any established infrastructure for diving here, visitors must be completely self-sufficient. You need compressors, generators, as many spares of everything as you can realistically carry and a very generous supply of drinking water.
Any visit has to be planned as an expedition. A 4x4 vehicle is required, and you must stick to the tracks and be prepared to cope with accidents, breakdown, illness or injury.
The Nullarbor Plain is no place for the faint-hearted, but it is a very special place. It is the largest area of continuous limestone on Earth, and explorers have made only the smallest impression on the caves to date.
It has the potential to become as important to underground activists as the Himalayas are to mountaineers.
While the surface is hot, dusty, monotonously featureless and flat, the situation below ground is quite the opposite. Ninety metres beneath the barren plain lies a scientific and sporting frontier of immense challenge.
For travellers on the Eyre Highway displaying their Ive Crossed The Nullarbor car stickers, this empty place is notable only in that it links the states of Western and South Australia.
The only real indicators of what lies below are the rare crater-like holes indicative of major collapse deep underground.
These huge, sheer-sided pits or cone-shaped depressions are usually blocked at the bottom with boulders and debris but occasionally, as at Murra El Elevyn, Cocklebiddy and Weebubbie, an opening may lead on, down into darkness.
What then follows will astonish any first-time visitor. The immensity of the underground tunnels is almost unbelievable. Massive boulders are frequently covered in inches of dust - these caves are a time-vault.
Not only do they indicate enormous, long-abandoned waterways - evidence of a different climatic regime - but they are the repository of scarcely imaginable wonders.
In one deep cave alone, some 14 new species of mega-fauna have been discovered, including giant kangaroos and other huge marsupials long since extinct.

WEEBUBBIE AND COCKLEBIDDY are among the largest tunnels in the world, and predictably lead to enormous, clear subterranean lakes and ongoing flooded passages. Their size and elemental darkness can give you a sense of agoraphobia, and strict control of the imagination is required.
Given the geology, it was clear from the moment that the first explorers chanced on these sites that the deep subterranean waterways were contenders for the longest underwater caves in the world.
In 1983, little more than 10 years after the first tentative explorations, Cocklebiddy was established as satisfying that claim.
In recent years the site may have relinquished top place, but exploration has now extended the overall length to more than 5km. For would-be visitors, no matter how limited their objective, this is an exceptionally physical and mentally demanding project. Not surprisingly, only a handful of people have ever been to the further reaches.
Tommy Grahams was one of the earliest caves on the Nullarbor to be explored, and the boulder-blocked end here is only two sumps into the system, so it seems to be a site well worth looking at.
We are told that only 50-60 people have reached the furthermost point, which seems very surprising. There is little doubt that this cave could run for many kilometres if the boulder blockage could be passed.
We know what to expect - two dives totalling about 350m of submerged passage, separated by an enormous air chamber where Aussie mates have warned us about a disturbingly high concentration of carbon dioxide.
I am carrying the crowbar, strapped to a side-mounted cylinder, and as I struggle up and over the hill towards sump 2, I can hardly breathe.
With my short legs the bloody thing is dragging on the floor, and catching on boulders as I pass.
Increased CO2 is very stressful. Being able to breathe is basic to life, but ones psychology is also extremely important in these places. Yes, it is very tempting
to breathe off your cylinder, which solves the problem, but then there is a heightened sense of dependency, and this induces a different stress - namely, the fact that now you only have so many minutes of air-supply left!
The real answer lies in composure, in controlling ones breathing, but this, I have found, comes only with experience.

AS WE FLOP BACK INTO the warm crystal water to continue the penetration there is a huge sense of relief. Fresh air and weightlessness restore the balance.
We reach the end of the cave, remove and stow our diving equipment, and set about searching for the way on, once more in elevated levels of CO2.
In no time I am forced higher and higher, up and into a mass of loose, friable debris. A tight squeeze forces Steve to hold back; being small like me is now the greatest advantage.
Judicious removal of rocks requires the utmost caution. The cave demands that I ascend, and the enticing voids are all there beckoning.
At about 23-25m above water level I can see into a substantial chamber that has never before been entered - who knows what might lie beyond
But as I lie between the rocks, my heart thumping, I know that I have reached my limit; my nerve has ebbed. Just one small destabilising movement, and it would be game over.
Exploration in a hideously precarious boulder-fall like this is like a game of chess, and today the outcome is stalemate. Next time the moves will be known... next time.

FABULOUS DIVE SITES are scattered far and wide on the Nullarbor. We visit Nurina on the Roe Plain and explore short sections of new passage both here and in Murra El Elevyn. Murra and Tommys are both magnificent recreational dives, cave-diving classics on the international stage.
We spend three weeks probing various sites in this immense area, divulging short extensions at most sites. Our final dive is purely recreational and photographic, to Weebubbie, a place that could never disappoint, no matter how many times one returns.
Weebubbie is like some long-abandoned giant quarry. The sun-baked walls are loose and overhanging, a rich orange-brown colour, and bare of any vegetation. To the unprepared this is not an easy place to access but, as ever, our hosts have come well equipped.
Using a hauling-frame, loads can be despatched some 30-40m vertically by rope, and recovered later by hitching up to a 4x4. A dive here is still a lengthy, hot and sweaty process, but this must be the best site of its kind in Australia.
From the far end of an enormous 140m-long lake, the tunnel descends abruptly on a steep boulder slope. Hundreds of metres of cave are then traversed at relatively shallow depth.
We break out from normal-sized tunnels and a breath-taking expanse of bouldery terrain stretches far beyond the beams of our torches.
This is the biggest tunnel I have ever seen, and I am conscious that it would be so easy to lose sight of that almost-invisible dive line.
Losing a sense of direction here doesnt bear thinking about.

THIS IS A GHOSTLY GREY, unforgiving wilderness that demands the utmost respect. My camera flash fires again and I can only hope that the remote sensors are angled correctly at this instant and can pick up the flash at such a distance.
Seeing this incredible passage illuminated by such powerful lighting is overwhelming. Nothing can detract from the wonder of this place, but capturing just one decent image will make this trip worthwhile.
The psychological adjustments for diving the Nullarbor are daunting. The discomforts begin with the physical remoteness, heat, dust and flies, and are compounded by inability to wash and lack of contact with the outside world.
The weather can be extreme, too. There may be no rain for many months, and yet within half an hour a localised storm can drop an average years fall.
The autumn months, April and May, are perhaps the most pleasant, while the water temperature in the caves is about 17°C all year round.
There may be flooded passageways deep underground, but the water is saline and undrinkable.
It seems almost bizarre to think that when you leave the surface world and set off into the enormous cave tunnels with all your dive gear, you also have to carry enough drinking water for the day!

CDAA QUALIFICATIONS
The Cave Divers Association of Australia offers four certification levels:

CAVERN:
Similar to other global programmes: depth limit 20m, penetration limit 40m.
SINKHOLE:
Similar to above, but depth limit 40m.
CAVE:
Depth limit 40m, penetration limit one-third of twin cylinders. Passage size must allow for buddy-breathing side by side.
PENETRATION:
Depth limit 40m, but encompasses smaller passages and stage diving.
Training programmes begin with Cavern, often run in conjunction with Sinkhole. To access the programme, divers must have held an open-water certification for more than 12 months and have undertaken at least 15 dives, including two night dives. www.cavedivers.com.au