It is the polluted air that Lieutenant Alastair Harris notices. Not on board his submarine, the mighty HMS Vanguard, which stretches the length of two football pitches, but when the boat — never the ship — docks and he breathes “fresh” air for the first time in months. After the carefully filtered stuff that fills his lungs while Vanguard is underwater, the air the rest of us share tends to smell a bit.

Harris, 27, has been in the Royal Navy for four and a half years, two of them in submarines. Although his father was a submariner, it was never his intention to tread the same path. His economics degree from Durham University took him initially into accountancy but office life was not for him: “I wanted something more adventurous.”

He joined the Navy in general service and it was not until a submarine “acquaint” course — a short taster — that he began to see the attraction. From then on he was hooked. “There was a real sense of camaraderie and I began to see with my own eyes the level of professionalism that was required.” There was also the excitement that comes from guiding 16,000 tons of nuclear submarine round the oceans.

Harris is a warfare officer, which means that when the sub is underwater he uses his navigation skills and equipment in the dive control room to help to keep Vanguard and its Trident missiles pointing in the right direction. When the boat is docked, at its base at Faslane on the Clyde, for example, he and his fellow officers share responsibility for the submarine, everyone on it, including visitors and contractors, and all weapons and machinery.

At sea, on the surface, he takes his turn as an officer of the watch. Reporting directly to the captain, he will have sole responsibility for the safety of the 150 crew — the number may vary depending on what the sub has been dispatched to do. He has yet to fire a shot in anger and says: “God willing, we will never have to. But we are there and trained for it if we do.”

Only the amount of food on board limits the time a nuclear submarine can remain at sea. Of his first operational trip — more than 60 days, although he cannot disclose exactly how long — Harris says: “I enjoyed it, although it’s a demanding job and it keeps us busy. I suppose the real issue is very limited contact with family and friends.”

Submariners can receive short “familygram” messages from home but cannot respond because to do so could reveal the boat’s position. That sounds tough, not only for the crew but for wives and girlfriends too.

Incidentally, submariner is a job for the boys. The service is closed to women. The reason given by the Navy is not to do with shared quarters, confined spaces or the risk of radiation, but contaminants in the sub’s atmosphere that could harm a foetus.

Some things about being in a submarine sound deceptively normal. In recreation periods the crew watch films, use exercise machines or take part in games and quizzes. But the harsh reality of being in a long, thin tube in the yawning chasms of the deep for weeks on end is that help is a long way off.

“We are all aware that what we do is not ordinary,” says Harris, “and we are also aware that if something goes wrong we need to be able to sort it out. But that’s one of the attractions for me.”

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