In 1938 and throughout the Second World War, guests at a luxurious hotel in London unknowingly slept beneath a floor full of explosives. St. Ermin’s Hotel was, at this time, the home base for secret British spies and the wartime intelligence community—yet guests staying there had no idea.

St. Ermin’s was constructed as private mansions on the site of a 15th-century chapel in 1889. A decade later, the mansions were connected and the building transformed into the hotel it is today—a hotel that happened to have a very strategic position: centered among all the wartime intelligence offices and close to the Houses of Parliament. In the ramp up to World War II, the British government knew it needed a place to call home for wartime operations. The hotel soon became the place to meet and discuss intelligence information. Spies and officers would come here to exchange information, train new agents, and even conduct job interviews in the various rooms upstairs.

In 1938, the British Secret Intelligence Service Section D moved in, taking up the top floors of the hotel. This department was comprised of demolition agents who would eat and drink in the bar, then head upstairs to plot against Germany. And they kept their namesakes close at hand: a stockpile of explosives stashed right in the hotel, above the very rooms in which sleepy guests retired for the night.

“Even those who work in the closed world of secrets must have a base of operations, and what better place to hide in plain sight than the iconic and elegant St. Ermin’s Hotel,” espionage writer and editor Mark Birdsall wrote in the foreword to House of Spies, a book by Peter Matthews about the espionage connection to the hotel. “I hesitate to guess the number of people connected to intelligence and covert activities that have passed through the foyer of the hotel through the years, or walked its secret corridors on to the streets of London.”

Today, guests to the hotel can fully immerse themselves in the spy experience. A number of items are on display from the espionage years, including a silk scarf printed with radio codes necessary to send information back to the UK from Occupied France (silk was light, easily packed, and quick to burn if the information was compromised); an original hotel rug from the spy era, oddly found at a market in Istanbul; a Division Bell, which connected to the Houses of Parliament (it’s still connected) and alerted members of Parliament to a vote—they then would have only ten minutes to walk back to the House of Commons to place their vote; and a cabinet filled with historical memorabilia dating to pre-1900. Kids can get in on the fun, too. At check-in they receive a top-secret spy packet just for them, with “ultra-secret code red” clearance, which takes them on a historical tour of the hotel and includes a trivia game.

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There’s something special in store for guests occupying the top two floors of the hotel, too: the knowledge that they’re staying in the former official headquarters of MI6, or Military Intelligence, part of the British Secret Intelligence Service. During World War II and after, MI6 used these floors as their base, welcoming senior personnel and infamous double agents—like Guy Burgess and Kim Philby, both of whom worked for the secret service but were also Russian spies.

Burgess did much of his work from within the hotel’s Caxton Bar, as well, handing over top-secret government files to his Russian contacts there. But that wasn’t the only espionage the bar saw. Intelligence officers Ian Fleming and Noel Coward were often seen there—and according to Matthew’s book, Winston Churchill likely devised, over a couple glasses of champagne, the notion to have a Special Operations Executive there during the Second World War. The special ops team began in three rooms on the second floor of the hotel.

Though the historic hotel still stands, its espionage days are long gone, with only artifacts and history as a reminder. Well, at least as far as we can tell you…otherwise, it’s classified information.