By James Shaw
BBC News at Faslane Naval Base
The BBC is given a glimpse inside a Trident submarine ahead of the future of the fleet being put to the vote in Westminster.
Patrols last up to 90 days, few of the crew know their location
Climbing down the shaft of a Trident submarine's escape tower into the body of the vessel is like entering another world.
One of narrow, low passages, cramped living quarters and working areas which are a dazzling array of computer screens and control panels.
During a routine 90-day patrol, the interior of the sub is the only world the crew know.
Just a few of the most senior officers on board are allowed knowledge of their location.
They will not surface, except in an emergency and they are not allowed any communications out of the boat.
All this so that the deterrent effect of the Trident fleet is not compromised. Because this man-made world includes not just a nuclear reactor, but also a clutch of some of the most deadly weapons on the planet.
Deterrence theory dictates if Britain's nuclear weapons cannot be located by an enemy, they cannot be destroyed. Therefore that enemy would not risk an attack, knowing there was a real risk of retaliation.
Finger on the trigger
The theory becomes absolutely tangible in the weapons control room, in the shape of a plastic pistol grip connected to a length of flex stored in a safe. It turns out the big red button is a myth. Instead there's a small red trigger on the pistol grip which would launch the submarine's missiles.
This arrangement allows the weapons officer to check other data from his control panels while holding the nuclear trigger in his hand. Only he and his deputy know the combination which opens the safe. The number is not written down, simply committed to memory.
The trigger can only activated if the Captain turns a key on a panel in the control room.
Like all Trident captains, Commander Paul Dunne, is acutely aware of his responsibilities. Particularly as he will shortly begin his first patrol in charge of HMS Vigilant. He says the sub's isolation and the need for total self-reliance are the hardest challenges for the crew: "If something goes wrong, they have to fix it. And so from that aspect it's a huge responsibility".
There are some lighter moments and chances to relax. Commander Dunne has more than 60 hours of music on his ipod, including the Keane CD, "Under an Iron Sea".
Some Trident crews have spent breaks creating eighteen-hole crazy golf courses through their vessel's corridors, ladders and bulkheads.
Further back along the length of the hull is the missile room. The biggest space in the whole vessel, stretching almost to the stern. There are two rows of massive metal cylinders as thick as tree trunks, capable of holding the submarine's maximum payload of 16 Trident missiles.
To one side are a dozen or so cots. This is where trainees sleep when the boat is at the quayside.
Weapons submariner Lee Wilson is just 22. He says he loved the job from his first patrol at the age of 20.
The social life on board includes bingo and film nights and he doesn't worry too much about the moral questions around the job.
"I feel that we're keeping the country safe," he says.
If the sub ever did receive an order from the prime minister to launch its missiles, the Captain's command to fire would be broadcast throughout the vessel, no doubt in an atmosphere of hushed anxiety and tension.
And however the small world of the vessel coped with that situation, the world outside it might in those few moments be utterly changed forever.