By Giles Dilnot
HMS Vengeance is part of the United Kingdom's independent Nuclear deterrent, armed with 16 Trident II D-5 missiles carrying nuclear warheads of varying firepower.
Beneath the oceans of the world, somewhere, silently and undetected, lies a British submarine carrying 16 nuclear tipped ballistic missiles and up to 160 men.
Its role is very straightforward: to maintain a constant readiness to unleash nuclear retaliation if the order ever comes.
The aim: to deter any pre-emptive attack on the United Kingdom.
Trident subs are 30 yards longer than a football pitch
The Royal Navy has four of these Vanguard class boats and one always has to be cruising the depths to provide a "continuous at-sea deterrent", the argument being that a weapon based on land would be too vulnerable to a surprise attack.
The vessels themselves are huge. Thirty yards longer than a football pitch, or 18 double-decker busses long.
But if the technology of both submarine and missiles are remarkable and very different from other branches of armed forces, then so is the crew.
The dolphin emblem is a badge of honour for the crew
They see themselves as the Navy's elite, the Dolphins on their uniform as prized as a paratrooper's wings.
As they rehearse their launch drill, the uniqueness of their role in the British armed forces becomes apparent as the guardians of the nation's nuclear arsenal.
Virtually incommunicado for three months at a time, each sailor receives only two 60-word "family grams" a week to which he cannot reply.
600 meals a day
The officers and crew operate in a non-stop cycle of six hours 'on' and six 'off'.
Life on board HMS Vengeance is cramped
In the downtime, the sailors grab some sleep, study for educational qualifications or take turns in the predictably tiny gym.
In such an environment, it is hardly surprising, as the commanding officer concedes, that the sub's chef is probably "the second most important person" aboard.
His role is to prepare four meals a day for the officers and ratings. With over 600 covers in a galley substantially smaller than your average domestic kitchen. And the fresh food runs out after a month.
The problems of storing food are not the only aspects of life on board which seem strangely reminiscent of Admiral Lord Nelson's HMS Victory, rather than what you might expect on a 21st century modern Vanguard-class submarine.
The crew are on constant standby as nuclear deterrent
The accommodation, wrapped around each of the missile tubes is beyond cramped.
The bunks are stacked four high, with 12 men in each tiny compartment.
No wonder that officers told us that tolerance was the single characteristic that submariners share.
Despite her sophisticated internal guidance system, nuclear-powered propulsion, stealthy design and an armament that a sailor at Trafalgar could never have envisaged, this man o'war, like Nelson's flagship, relies on the skill and resilience of a unique breed of seafarer to keep it patrolling the depths, somewhere in the world's vast oceans.
The Politics Show's film about life on board Britain's nuclear armed submarines is broadcast on BBC One on 13 December at 1200 GMT