The death of two British submariners has highlighted the extreme conditions endured by those who man the Royal Navy's underwater fleet. What is it like to live and work hundreds of feet under the surface for months on end?
Just the bald numbers tell some uncomfortable truths. Eighty-five metres long, 10 metres wide and about the same again in height - the dimensions of one of the Royal Navy's Trafalgar class submarines. Aboard which, for several months on end, live and work a crew of 130 men.
HMS Tireless, on which mechanics Anthony Huntrod and Paul McCann died in 2007, is just such a sub. On Tuesday a coroner ruled the men's deaths from an on-board explosion, had been caused by "systematic failures" of procedure.
Bunks are at least now a little more spacious than they were
The Navy has said it will learn lessons from the incident, but for many the most perplexing question is how people can willingly spend weeks at a time without glimpsing daylight, gasping fresh air or speaking to their loved ones - cocooned in a vessel about as long a football pitch and a fifth of the width.
Britain has a fleet of 12 submarines - all nuclear-powered. With the old diesel-powered subs, patrols could last as little as a couple of weeks. But today their range is effectively unlimited, meaning operations are often six months at a time.
Life on board is structured by an intense working pattern of six hours on, six hours off, for an entire operation.
Stifling claustrophobia is a clear occupational hazard, and likely sufferers are usually weeded out in the selection process. If not, they may need to be grabbed and sedated until they can be landed ashore, says Ted Hogben, a warrant officer who worked on nuclear-powered subs for 20 years.
Mentally, life on a submarine can be challenging and suits only certain temperaments.
"The Brits are well known for their phlegmatism which tends to help," says WO Hogben. "If you don't get on, you move on." This usually means transferring to the surface Navy."
FOOD, DRINK, TOBACCO
Much of the produce is frozen, bread is baked daily and most submariners agree the quality depends entirely on cooks' abilities. Four meals a day are served but most will only eat two because of shift patterns. Some say lack of exercise means submariners tend to put on weight.
Until 1970s, submariners enjoyed tot of rum at lunchtime and beer in the evening
Rum ration is no more but a half-a-pint is allowed.
Smoking used to be allowed in designated compartments but is now completely banned
To many, life on a submarine would seem not unlike prison, trapped in a confined space for a six-month stretch with 100 or so other men. Worst still, you are living with the daily threat of attack, drowning or fire.
"The first week is a bit of a drag, as is the last week when you are thinking about getting home. But in between, time flies by. You sleep, work, watch movies and so on."
Yet space is clearly at a premium, personal space all the more so. Most crew members have their own bunk, but juniors tend to have to take whatever's free - known as "hot-bunking".
The only personal possessions might be a family snap, books and DVDs. And there is a mess room for relaxation and recreation.
In the past, crew would wear civilian clothes to work in and sleep in, so they could be ready in the event of an emergency. They were allowed to remove their shoes at bedtime. Inevitably there was a strong smell of oil and diesel mixed with body odour.
Submariners today, by comparison, wear a uniform similar to that of a sailor. Bunks are a little more spacious and there are showers, toilets, and a small laundry.
Yet while efforts are made to preserve the normality of life in some quarters, in others it couldn't be more stark. Communication with families ashore is one of the chief examples of this disconnect.
A submarine can be reached anywhere in the world via satellite or through encrypted radio messages. But a submariner cannot.
Living and working conditions mean submariners form close bonds.
No matter whether a crew member's father is dying, his daughter is seriously ill or his wife has run off with another man, it is likely he will be the last to know.
While a family can get a message to the commanding officer on board, he may decide not to pass it on.
Captain Mike Davis-Marks, commanding officer on HMS Turbulent from 1996 to 1999, says he often had to make just such tricky judgement calls.
"If you are deep on patrol and can't be surfaced, I would invariably sit on it until we arrived in the next port of call," says Capt Davis-Marks.
"There's little point in telling someone bad news and making them fret about it. Sometimes they get annoyed with you, but you have to take a bigger view."
The pressure of being away from home for so long can take its toll on relationships.
Marriage breakdowns are not uncommon, and these days there are fewer marriage quarters at base ports, making it harder still.
Guide to a Trafalgar-class fleet submarine (2.43Mb)
Opens in a PFD docuement.
The average age of a submariner is about 21. All are male and most are of an age when they are still single. But married crew members say it takes a special woman to be a submariner's wife.
"She has to be a very different sort of lady. When I was away my wife used to do everything, from the electrics to the painting and decorating," says John Cumberpatch, a submariner from 1955 to 1964.
Jim McMaster, a submariner from 1965 to 1986, says you "have to learn to be satisfied within yourself and have peace of mind and confidence in your family."
So what does a submariner crave most during the long stretches away at sea?
"What any young red-blooded males crave," Mr McMaster explains, without elaborating.