Vengeance is her name and vengeance her only purpose

Here the debate over renewing Britain's nuclear arsenal is made real: 16 Trident missiles, each with up to 12 independently targeted nuclear warheads. Once airborne, the 60 tonne missiles travel at four miles a second. HMS Vengeance, like all Vanguard class Trident ballistic missile submarines, carries enough nuclear firepower to destroy 1,500 Hiroshimas within 15 minutes.

Resting at port at the HM Naval Base Clyde, the Vengeance resembles a prehistoric sea beast - flattened forehead, rusting spots like barnacles, squat, obsolete flippers from some abandoned evolutionary stage. A team of workers - their silhouettes framed against rolling, snow-capped hills - gingerly repair a few of the tiles on her giant bow, which have fallen off - there are 89,000 in all.

There are 1,600 people stationed at the base at Faslane to fuss around the Vengeance and her three sisters - the largest naval base in Britain, whose sole purpose is helping keep these doomsday boats at sea.

On board, 130 officers and men spend up to three months at a stretch at sea amid the wires, gauges, communication equipment, missiles and nuclear reactor of the Vengeance.

The ship's sole function is to rehearse, and execute if necessary, the launching of the missiles.

What strikes you immediately on board is the eerie hush. There are signs everywhere enjoining silence: "Noise kills". The nuclear reactor, which powers the enormous vessel, is no larger than a wheelie bin and is completely silent. The only noise is that of the ventilation system - the constant hiss of used air being filtered, treated with oxygen, then re-released.

One of Britain's four nuclear subs is continuously on patrol at sea, a routine that has been kept up for 38 years. According to deterrence theory, it is the ultimate weapon: even if all of mainland Britain is destroyed, the sub will be able to retaliate. The name says it all: Vengeance.

Although it is enormous by submarine standards - causing the same water displacement when under water as an aircraft carrier - space is at a premium. Men - there are no women - sleep in 6ft bunks, nine to a room. A sign on the washroom reads "no Hollywood showers": a continuous water supply is for movie stars.

This crowding has resulted in a strange intermingling of domesticity and destruction. Men shower in cubicles situated between the missile silos, jog on running machines positioned underneath them, and sleep in bunks a few feet from them.

Attached to each of the 16 onboard missile silos - massive, stolid, primed - is a detachable ironing board and an iron.

Despite this proximity, there is nothing to suggest intimacy with the weapons. While the first atomic bombs were given deceptively innocent sobriquets - Little Boy and Fat Man of Hiroshima and Nagasaki - the Vengeance's arsenal is known to the crew as Missile 1, Missile 2; that is all.

This formality speaks of the sobriety of the Vengeance's mission, and the precision with which Trident missiles have been designed to cause mass destruction.

"They say the missiles are accurate to within 6ft, but it could be 30ft, or a football field, or a city; they are still going to level the place," the ship's Commander, Mark Lister, says. "The responsibility is awesome and the scrutiny is continuous. We take our mission extremely seriously, and serve the Prime Minister, whatever his or her orders".

To the officers - many of them engineers - the missiles are machines; it is their job not to ponder, but guarantee their smooth operation. Even so, the quietest place on board is the upper compartment of the missile section. There, crewman can stand within a few inches of nuclear warheads and place their hands on the cool exterior of their casing. There are no ironing boards here, only gauges, pumps, hatches, and the strangely soothing hum of the ventilation system.

It has the feel of a prayer room. Sometimes, the captain will come and stand among the rows of missile tubes, eight on each side like trunks of stately redwoods. He calls this room "Sherwood Forest".

Firing drills take place twice a week. In order for a missile to be launched, the captain must insert a small key into a slot on his control panel and turn it from "hold" to "fire". The weapons engineer, in a different room, then pulls a trigger modelled on the pistol grip of a Colt 45 revolver.

Launch orders come directly from the Prime Minister.

In a safe in the captain's quarters is a handwritten letter from Tony Blair bearing his instructions for nuclear war should Britain suffer a first strike that kills millions of people, including the government. The orders are to be opened and acted upon only if the Vengeance fails to receive communication for a set number of days and has other reasons to believe Britain has been destroyed. The contents of the letter are classified.

If everything is running smoothly, submarines are remarkably healthy places to work.

The air is pumped full of oxygen and the reactor is so well shielded that a three month underwater tour exposes a crewman to less radiation than a long-haul air flight. The flip side is that if something does go wrong, it is likely to be catastrophic. Every crewman wears a dosimeter on his hip to measure radiation exposure. There is a sign tacked up outside the on-board medical quarters: "God's Waiting Room".

The biggest threat to the crew on the long underwater tours is to their mental health. Life on a submarine is a potent mixture of the claustrophobia of cramped living quarters and the agoraphobia of being surrounded on all sides by a seemingly endless expanse of ocean. For some, the pressure is overwhelming.

"Men get 'coffin dreams'," Lieutenant Dan Wright explains. "They wake up thinking they are stuck in a coffin. The feeling usually passes in a few minutes. I've heard that some men lose it on board, but I've never witnessed anything more than the nightmares".

The crew work through their duties in a state of timelessness; with no windows and no natural light, the only indication of the time is the next meal. The menu is the same every week - curry on Wednesday, fish and chips on Friday.

DVDs, books and board games are the preferred distractions. One of the games in the officers' cabinet is "Risk: The Game of World Domination". Some take work towards Open University degrees. Once a week, each crewman can receive a 40-word message from his family, sent on a page with 40 boxes.

Although the captain will inform the crew in the event of a launch - "we take everyone to war with us," he says - not everything is shared. The captain vets personal messages. One that tells of a death in the family will be withheld from the crewman until the boat returns to port.

At the officers' mess, men don't mind discussing the geopolitical debate to which they are now central. Universally, they believe their submarine should be replaced with a boat capable of both tactical strikes - nuclear bunker-busters, say - and strategic deterence; a doomsday boat capable of nuclear-lite.

Hardly any of these men are old enough to have served during the brinkmanship of the Cold War; Cmdr Lister is 45. But to everyone on HMS Vengeance, the prospect of nuclear war remains only a key-turn away.

"With all the talk about emerging threats, the question of whether these weapons will be used is still relevant," Cmdr Lister says.

There is a terrible beauty to this submarine, the brain-child of a generation's brightest minds. It highlights a central issue for humanity: whether the gift of consciousness may also be our curse.

Somewhere in the depths of the Atlantic, the missiles of a Royal Navy submarine are silent and waiting.