The underwater engineers who make pressure their business

The hazards of the sea 100 metres down hold little fear for the specialists who ply their trade there. "It's a peaceful environment, if you can believe that," said Julian Thomson, a former Royal Navy diver now working for the company whose divers were able to penetrate the Kursk.

The hazards of the sea 100 metres down hold little fear for the specialists who ply their trade there. "It's a peaceful environment, if you can believe that," said Julian Thomson, a former Royal Navy diver now working for the company whose divers were able to penetrate the Kursk.

The 12 "saturation divers" from the North Sea company Stolt Offshore, aged between 35 and 40, and volunteers for this mission, have worked at twice this depth. To get a licence they had to descend to 100 metres during training. "What they did on the sub is what they'd do every day of the week," Mr Thomson said.

All were engineers before they embarked on their lucrative diving careers on North Sea oil rigs. Mr Thomson said they were essentially handymen: "The diving is just a means to get a technician to his work site."

Before any dive, these technicians must slowly acclimatise to the pressure below. They can spend up to 30 days in a compression chamber on the support vessel, travelling back and forth in the pressurised diving bell to the job. After leaving the wrecked hull of the Kursk they will spend four days decompressing in the chamber.

The dangers facing the crew, eight of whom are British, are similar to scuba-diving, but more extreme. Oxygen tanks are no use below 30 metres, because any nitrogen in the blood can form life-threatening bubbles at this pressure, and pure oxygen becomes toxic.

Saturation divers therefore breathe a mixture of pressurised helium and oxygen. The most obvious side-effect is that the divers' voices become squeaky. "They sound like Donald Duck on a bad day," Mr Thomson said. The divers' microphones are patched through helium speech "unscramblers" so they can be understood.

Their air is fed from the vessel through umbilical cords. The tanks they carry on their backs are for emergencies only, in case the cords puncture. The hoses also contain the cables connecting to the lights, cameras and microphones on their helmets, and hot water.

At the Kursk the temperature is about 2C. Each diver is out for six hours at a time, so keeping warm is essential. The hot water is pumped between the two layers of their neoprene wetsuits. It feels like a warm bath, Mr Thomson said.

Beyond the age of 40 even a fit body cannot take much of this style of working, and divers must retire. They will have worked a pattern of three or four weeks on, then two weeks off.

But the rewards are great; a successful saturation diver can earn up to £80,000, the top ones even more. But it is a very dangerous way to make a living - a momentary lapse of concentration can prove fatal.