BREATHE in, breathe out, breathe in, breathe out. It's something we take for granted, in fact it's so natural we can do it in our sleep. Live between sea level and 10,000ft and, as long as you are fit, there are no problems.

Climb above that level or go beneath the sea and we need the assistance of oxygen to survive.

That's something the 118 crew of the stricken Russian submarine Kursk will be focusing their minds on at the moment - although if the air is getting thin they won't be focusing their minds at all.

The sub has been lying 325ft down on the bottom of the Barents Sea since Saturday and if the reactor is shut down so will the oxygen-generating machinery. By now the crew is likely to be suffering from the effects of hypoxia or oxygen deprivation. But those effects are not as you would imagine.

Some years ago I underwent training in a decompression chamber before being allowed to fly at altitude in a high speed jet.

The RAF was anxious I recognise the effects of hypoxia in its early stages so I could take action before it was too late.

The submarine-like contraption I was placed in contained no oxygen and I wore a pilot's mask. I was asked to take it off and complete some simple tasks, such as writing down the alphabet backwards.

I only reached X before my mind became clouded, like I was in a dream world, unable to concentrate. After a minute I was incapacitated and couldn't even be bothered to try the tasks set.

Ninety seconds passed and I was told to put on my mask. The most alarming thing was that I didn't really want to. The first thing to go was fear. I couldn't have cared less even though I was told in no uncertain terms that I was dying.

It didn't feel bad, there was no sensation of suffocating. I was still breathing in and out as normal, a gas was present, just not the right gas to keep me alive. In fact at one point I started laughing uncontrollably, like I was drunk.

If the air is running out for the submariners of the Kursk, the chances are they will no longer be suffering. They will have undergone all the anxiety, panic and sadness about the prospect of dying when the air was rich in oxygen and their heads were clear.

The big problem now is that if they are hypoxic they will be unable to aid the rescuers. Even opening up the hatch from the inside will be beyond them. They probably won't care if they are rescued or not and if they are bothered, the thought processes required to act will be beyond their ability.

Roger Mallinson, 62, can imagine exactly what they are going through. He and a colleague were trapped in a submersible on the bottom of the Atlantic in 1973 for three and a half days. They had been working to bury a telephone cable 150 miles south east of Cork when their machine, Pisces III, crashed out of control. Other deep sea craft were able to attach a line to the damaged craft and it was eventually hauled to the surface by the cable ship.

Their plight landed them in the Guinness Book of Records for the deepest underwater escape - 1,575ft.

"I never thought we would survive," says Mr Mallinson, of Troutbeck Bridge, Cumbria. "We never spoke about death when we were down there. This has brought the memories flooding back.

"They will all be in it together. Nobody's any different any more, there will be no one pulling rank. I just hope they come out of this alive. I would think the men will already know the chances of survival are negligible. I hope I am wrong. I feel so sorry for the crew. Someone needs to get them out fast."

If they don't, the Kursk will go down in history as the world's worst submarine disaster.

That claim was once held by a British vessel, His Majesty's Submarine Thetis, which sank during diving trials on June 1, 1939 with the loss of 98 hands.

There were 103 men on board Thetis - twice the size of the normal crew because of the observers from the construction companies. She was accompanied by a tug to the dive site 38 miles out of Liverpool.

But when she reached the dive site, Thetis refused to dive. Her tanks flooded as they should, but her stern was still not heavy enough to sink. Suddenly, her bow dropped to the sea floor about 140ft down and stuck in the mud.

Someone opened the No 5 torpedo tube and the sea flooded in. A third of the Thetis' hull was filled before the water could be contained. The men inside had just ten-12 hours of air left.

Four of the crew managed to escape. The other 99 asphyxiated.

Thetis, though, was re-born. She was returned to the Cammell Laird yard at Birkenhead where she was re-fitted and re-named HMS Thunderbolt.

Four years and a day after the Thetis went down, the Thunderbolt was hit by a depth charge in the Mediterranean. All 57 crew died. She remains the only British submarine to be lost with all hands - twice.

The Kursk is only the latest disaster in a chequered history which has seen 121 incidents involving Russian subs in the past 40 years.

In 1989, 42 crew died when a nuclear sub sank in the same Barents sea.

And with the Russian navy surviving on a shoestring and its machinery falling into decay, it will be a long time before the country's submariners will be able to breathe easy again.