Darkest Hour, the new film starring Gary Oldman as Winston Churchill, shows the iconic leader in some famous places familiar to plenty of Anglophiles and history buffs, locations like 10 Downing Street and the Houses of Parliament. But a portion of the film also takes place in a lesser-seen though just as history-soaked site: a stuffy bunker, the secret underground location of Churchill’s World War II operations from 1940 to 1945.

Today, that bunker is a popular tourist site, Churchill War Rooms (called the Cabinet War Rooms during its use for World War II), part of England's Imperial War Museums.

The Cabinet War Room as it’s seen today within Churchill War Rooms.© IWM

As war dawned and Churchill took the reins after the period of appeasement led by his predecessor Neville Chamberlain, the humble underground government storage space was hurriedly converted into a military information hub. Located underneath the Treasury building in Westminster, it covered about 3 acres and accommodated up to 528 Cabinet and supporting staff members.

The Darkest Hour crew spent three and a half weeks filming scenes that take place in the War Rooms, recreated by production designer Sarah Greenwood and her team at West London’s Ealing Studios.

Greenwood came to Darkest Hour as a longtime collaborator of director Joe Wright (Atonement, Pride & Prejudice). With the exception of 2015’s Pan, she has worked on every feature that Wright’s directed, plus two of his miniseries.

“We argue a lot," Greenwood tells Mental Floss, laughing, of their longtime collaboration. "We’re like siblings, actually. I’m like the older sister. We’ve been together too long.”

Gary Oldman and Lily James as Elizabeth Layton, Churchill's secretary, in the recreated Map Room in Darkest Hour (2017).Focus Features

Wright has spoken about how Greenwood often helpfully challenges his choices, but her plans for the War Rooms were an unusual instance of immediate agreement between the two filmmakers.

“I designed [the War Rooms set], drew up the rough plans and everything over a weekend, and when I showed it to Joe, he was just like—and this is very rare—he was like, ‘Yep, that’s great.’ There were very few changes that we made to that,” Greenwood says. “And I think that came from knowing what it was going to be like. Because we’d been to the real War Rooms, we knew what we were trying to capture.”

Greenwood, along with other members of the art department, visited the Churchill War Rooms about half a dozen times. She remembers being most struck by how apparent it was that this all-important nerve center of war operations was “cobbled together at the 11th hour, [with] furniture brought in from home. There’s this forest of beams from when they brought in a Naval architect to shore the whole thing up when they realized that it was not bombproof.”

The recreated typists' bay, on the set of Darkest Hour at Ealing Studios.Sarah Greenwood

Greenwood noted the War Rooms’ contrast to the Nazi sites for World War II operations depicted in the 2008 film Valkyrie: “It’s very sharp and organized and clear and cold colors,” she says.

“One of the most important things to understand about the Cabinet War Rooms is they’re an extremely improvised space," Ian Kikuchi, senior curator, Second World War at Imperial War Museums, tells Mental Floss. "The war is not necessarily a surprise, but the War Rooms are not a lavishly purpose-built facility. You can see its kind of lack of bombproof-ness everywhere you go, especially when you look up into the ceiling and you can see the gigantic layer of concrete that they had to add to the ceiling in order to try to improve the protection.”

The closest the War Rooms came to being directly hit was in September 1940, when a bomb fell on Clive Steps, leaving a small crater near what is now the visitor entrance to the site.

The Map Room as it’s seen today within Churchill War Rooms.© IWM

“It was a stroke of luck, really, that the War Rooms were never hit,” Kikuchi says. 

The film brought to bustling life a space that Kikuchi and his colleagues are accustomed to seeing frozen in time. 

“It was a real thrill actually," he says. "These corridors that I’m so familiar with—to suddenly see them on the big screen—I was really struck at just how right it all felt.” 

However, Kikuchi did, of course, recognize any deviations from reality that Darkest Hour made with its set, the most noticeable being a rearrangement of the rooms. For example, in the film, the BBC equipment room is right next door to Churchill’s underground bedroom, where he delivered four wartime speeches. At the real site, the equipment that transmitted these speeches is further down the hallway. 

Gary Oldman, as Winston Churchill, delivering a speech at the desk in his bedroom in the Cabinet War Rooms.Focus Features

Greenwood intentionally took some artistic license with the layout of the War Rooms, creating a more labyrinthine feel, unlike the real-life stretch of rooms along a long corridor. 

Within each room, though, the art department meticulously recreated the environs of that wartime bunker. 

Darkest Hour's Map Room and "beauty chorus."Sarah Greenwood

Though items from the 1940s tend to be readily available to filmmakers, Darkest Hour’s art department custom-made several props, since the technology and furniture in the War Rooms is so distinctive (and recognizable to the tens of thousands of tourists who visit the site each year). The telephones in the Map Room weren’t a simple, standard black; the so-called “beauty chorus” were bright reds and greens, color-coded and connected to a specific military department or intelligence service. Graphic designer Georgina Millett recreated whole wall-spanning maps specific to the era after several research trips to the British Library. 

The props team also built a replica of the wooden chair that Churchill sat in during tense meetings in the Cabinet Room. On a visit to the War Rooms with fellow cast members, Oldman had the rare privilege of sitting in the very chair from which the iconic leader conducted these meetings. “That’s something that you normally have to be a president or prime minister to get to do,” Kikuchi says. 

The Cabinet Room, at Ealing Studios.Sarah Greenwood

Today, on the ends of that chair’s armrests can be seen scratch marks, evoking the nervous energy of its occupant. Close-up shots in Darkest Hour depict Churchill making those gouges with his right-hand fingernails and with the signet ring on his left hand.

Darkest Hour also required some imaginative mystery-solving, alongside the historical research.

“One thing that nobody [among the historical consultants] would ever say, or couldn’t ever give us a real answer on, was whether there were tunnels linking 10 Downing Street to the War Rooms,” Greenwood says. “‘We don’t know' was the answer. I think it’s still a secret actually. I personally think there were tunnels between Downing Street and the Houses of Parliament and everything. But of course, nobody will admit to it.” 

The War Rooms' switchboard, recreated for Darkest Hour (2017).Sarah Greenwood

So Greenwood and her team created a dimly-lit set of tunnels stretching from their War Rooms to a small elevator that lifted Churchill back into the famous home of Britain’s Prime Ministers.

Since Darkest Hour takes place over the course of less than a month, beginning in early May 1940, the film doesn’t capture what it was like to spend prolonged amounts of time in the War Rooms. Sleeping in the cramped, rat- and cockroach-infested sub-basement (called “The Dock”), never shown in the film, was a necessity during periods of intense bombing for all but higher-ranking officials (who had bedrooms on the upper levels). Later during the war, 12-hour shifts underground meant that some staff members went weeks without seeing daylight.

Ben Mendelsohn, as King George VI, with Oldman as Churchill in Darkest Hour (2017).Jack English/Focus Features

But Wright and cinematographer Bruno Delbonnel still crafted a space with a clear lack of natural lighting, in contrast to the film’s scenes that take place aboveground during daytime, where large, bright beams of sunlight stream into screen versions of 10 Downing Street, the House of Commons, and other historic locations. The stark sunlight also evoked the weather of Churchill’s first month as PM, which was one of the hottest Mays on record. 

Today, the Churchill War Rooms bear an unassuming, modest entrance that’s easy to miss, though it has become an ever-more popular tourist destination since its opening as a museum in 1984. In 2017, the Churchill War Rooms welcomed over half a million visitors, “a number that I’m sure would amaze anyone who ever worked there,” Kikuchi says. 

Map Room Officers at work in the Cabinet War Rooms, 1945.© IWM

And as for what Churchill himself—a man who has eloquently written and spoken about the importance of studying history—would think if he could see the Cabinet War Rooms as a popular tourist attraction today? Here’s what Kikuchi had to say: 

“Churchill was a man born in the 1870s. There’s all manner of things about life in the 21st century that he would be amazed and baffled by. Churchill, in his memoirs, he talks about the moment of becoming prime minister, feeling like he was ‘walking with destiny.’ He was a man who was very conscious of his place in history. And I think he would be proud and gratified that his War Rooms still exist and are reminding visitors from around the world of that crisis moment in 1940 that you see dramatized so effectively in Darkest Hour.”

Darkest Hour is in U.S. theaters now and will be released in the UK this Friday. Churchill War Rooms in London is open daily.