Britain's fleet of submarines have been silently operating below the waves of the world's oceans for decades, and over Christmas that duty does not waiver.
While most people at home are enjoying time with friends and family, submariners on patrol over Christmas have little or no contact with the outside world.
Former Commander Ryan Ramsey said Christmas day on patrol is just like any other.
The 48-year-old, who retired after 25 years in the Royal Navy, said: "The day doesn't change - it is like every other day when you are on patrol except it is Christmas.
"The reality is sonar operators are going to be listening to the sonar, the ops officers are going to be doing operations ... and there is no change to the watch-keeping cycle."
The general watch-keeping system involves a pattern of six hours on, six hours off.
"If you are in contact with an enemy submarine or doing surveillance you don't have to have everybody up at the same time... that is what gives the endurance for submarines," he said.
Mr Ramsey was captain of the now decommissioned HMS Turbulent before his retirement in 2012.
The Trafalgar class, nuclear-powered attack submarine did not go without any festivities. The crew will often take decorations with them if they are serving over Christmas to decorate the mess, he said.
A church service is also held on board.
"If I think back to the last patrol I was at sea over Christmas, there was a really great feast - the chefs did an amazing job."
Patrols range from 60 to 120 days with the hardest thing, Mr Ramsy said, saying goodbye to loved ones.
"There is no internet - you receive a message which tells you what the news is maybe once a day, maybe once every three or four days."
Every couple of weeks sailors receive a "family gram", a letter containing 30 words from their family or friends back home.
Mr Ramsey handed over command of HMS Turbulent in December 2011.
The hunter-killer boat fired missiles during operations in Libya and was deployed on patrols in the North Atlantic and the Far East.
Mr Ramsey described a submarine patrol as "about 70% boredom and 30% pure adrenaline" - switching between the two in an instant.
"If you are in contact with an enemy submarine, I used to find that the most exciting stuff, because basically, it wasn't about the machine; it was about you versus whoever your enemy is," he said.
"They might be as good as you, and it is like chess, effectively."
He said the best part about submarines is you can "fight an entire war underwater without having any massive political effect that goes with it".
But he said the less exciting tasks involved crossing an ocean-going in one direction at one speed - with "nothing really happening".