Nicknamed 'the silent service', nuclear submarines are used to patrol the oceans and seas – armed with numerous weapons and capable of spending months underwater.
However, one question has been grappled with for decades. What do you do with a nuclear submarine when it has gone out of service?
Well, the UK is attempting a world first – to dismantle and cut up one of the vessels.
The Submarine Delivery Agency (SDA) is responsible for dismantling 27 nuclear submarines – 21 of which are already decommissioned.
Seven are stored at Rosyth in Fife, with a further 14 at Devonport, Plymouth.
The dismantling process has already started on three defuelled submarines at Rosyth – HMS Swiftsure, HMS Resolution and HMS Revenge – with about 90% of the overall metal free from radioactivity.
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But most of the radioactive parts are pipes, pumps and other metalwork in the area closest to the nuclear reactor.
Christine Bruce, SDA's assistant head of Nuclear Liabilities, told Forces News turning a nuclear submarine "into razor blades and tin cans… is quite a complex process and it takes quite a long time".
"Stage two is the next bit and that's the most challenging part, and in some ways, it's the most exciting part," she said. "It's when you take out what's left of the centre of the reactor.
"It's called the reactive pressure vessel and really, it's just like a giant kettle, but it's the most radioactive part that's left, so taking that out is key to the rest of the submarine being able to be disposed of like any conventional ship.
"That process has been quite a long time in planning because it's a large piece of equipment and it's quite radioactive and our absolute number one goal is safety."
She went on: "We spent quite a long time about 10 years ago looking at alternative methods and the method we're adopting is this dismantlement.
"No-one else has done that [and] when we manage to cut it up by 2026, that will be a global first.
"No-one else will have cut up a submarine," she added.