It's a story that's been mostly forgotten or unknown for decades... until now.
During the Second World War, Parliament had a secret war factory underneath the House of Commons. Workshops forming the 'Westminster Munitions Unit' were set up in the basement beneath Central Lobby.
Well, they weren't making bombs – it would have been far too dangerous. Instead, they were making parts for anti-aircraft guns to combat German V2 rockets and it was simply down to an idea that two clerks had to do their bit.
It followed one of the darkest periods of World War Two as German bombs rained down on the UK during the Blitz in the early 1940s.
The House of Commons chamber was destroyed and, from 1940-1941, the Houses of Parliament was damaged by air raids 14 times.
Thousands of workers at munitions factories across the country raced to produce parts for the war effort.
After some initial strong resistance from their bosses, the two clerks who wanted to set up a munitions unit underneath Parliament were successful.
Their persistence paid off and the Westminster Munitions Unit was created in 1943. It was fully operational by December of that year.
Forces News has been granted exclusive access to film in the vaults where hundreds worked without being able to tell a soul.
It's the first time Parliament has allowed access in modern times to see where the work was carried out and Forces News has been granted special permission to film.
Dr Mark Collins, Estates Historian at the Houses of Parliament, said: "It was a complete secret, absolutely nobody knew what was going on down here."
Today it's part of the central heating system complex and, looking at it, you'd have no idea that in the 1940s, millions of parts were being made to contribute to the war effort.
Dr Mari Takayanagi, Senior Archivist Parliamentary Archives at the Houses of Parliament, said: "I think it’s a really great unknown story that deserves to be better known."
Sir Lindsay Hoyle, Speaker of the House of Commons, told Forces News he had no idea the factory existed.
"I think the war effort, what people did, what sacrifices were made, we must never forget, because they all played their part, including the House of Commons," he said.
"Obviously they wouldn't risk explosives – they'd had enough with Guy Fawkes. I always presumed it was just the plant room.
"I look around and think this must have been the original plant, but far from it, here we are, a munitions factory. I've got to say, excuse the pun, but I'm absolutely blown away!"
According to records from the Parliamentary archives, it was on 1 June 1945 that a special BBC radio broadcast revealed its existence for the first time.
Millions of parts were produced by hundreds of men and women. Many volunteered their time, from MPs, their spouses, police officers and detectives and friends of those who worked in Parliament.
The main item produced was a torque amplifier which was a part of a predictor unit for mobile and stationary anti-aircraft guns. The amplifier was completed in time to be used to destroy flying bombs.
They also assembled detonator holders and priming fuses. More than two million shell fuse parts were inspected and 95,000 special packing case fittings were made as a sideline.
The Westminster Munitions Unit continued to operate until the end of 1945 to complete a contract for the Admiralty in connection with oxygen supply in submarines. Two hundred and twenty secret instruments were made for them.
Since 1945, little has been released about the unit and its story until now – it has remained largely forgotten.
And to the wider public, most people have no idea that there ever was a war factory underneath the Palace of Westminster.