Earlier this week, England’s Parliament announced that a secret door had been discovered in the walls of Westminster Hall.

BBC News reports that the 360-year-old passageway, located in the cloister on Westminster Hall’s west side, opens into a small chamber that would have led right to Westminster Hall if the other entry hadn’t been sealed. There are still traces of that entryway inside the passage, though, which include the original hinges for two wooden doors that would’ve been just under 11.5 feet tall.

Liz Hallam Smith, a University of York historical consultant for Parliament, explained that she and her team had been sifting through 10,000 uncatalogued documents about the Palace of Westminster when they uncovered old plans for the doorway, which they then located in person.

“As we looked at the paneling closely, we realized there was a tiny brass keyhole that no one had really noticed before, believing it might just be an electricity cupboard,” Smith said in a statement.

After several attempts, the Parliamentary locksmith managed to design a key that unlocked the door, revealing the long-forgotten passageway. Dendrochronologists analyzed wood from the ceiling and determined that the trees had been cut down in 1659, which tracked with historical accounts of the construction having occurred between 1660 and 1661 for the coronation banquet of Charles II.

According to Parliament’s statement, the passageway was used for coronations, Speaker’s processions—in which the Sergeant at Arms escorts the Speaker of the House of Commons from his apartments in the palace to the Commons chamber—and shortcuts by members of Parliament.

It hasn’t been used for decades, but it’s not completely empty: There’s a light switch and a working light bulb that historians believe was installed during renovations after World War II, and there’s also some cheeky “graffiti” from about 100 years before then. Bricklayers who restored the room in the years after the fire of 1834 scrawled “This room was enclosed by Tom Porter who was very fond of Ould Ale” and “These masons were employed refacing the groines [sic] August 11th 1851 Real Democrats” on its walls.

“The mystery of the secret doorway is one we have enjoyed discovering,” Mark Collins, a Parliament estates historian who helped find the passage, said in the statement. “But the palace no doubt still has many more secrets to give up.”

[h/t BBC News]