There are mystery books in a library but a couple of libraries in the United Kingdom have more mysteries to offer.

Behind a bookshelf in a 500-year-old home in Brighton, England bought by Freddy Goodall 34 years ago was a hidden passageway he discovered in 2021 while looking at an 1870 photo.

The passageway led to a secret room with doors that led to other rooms and tunnels, according to a report in the Smithsonian Magazine, quoting Goodall. The tunnels were connected to other houses in the community.

Meanwhile, a door deep within the lower floors of the National Library of Scotland is not open to the public.

BBC Scotland News reported that library officials found a small hatch on a wall behind filing cabinets in the 1990s and broke it down to reveal a passageway with arches into chambers and rooms.

The passageway used to be part of a street in Edinburgh called Libberton’s Wynd, which was demolished in the 1830s to make way for the George IV Bridge.

Almost two centuries-old remnants from the bygone era — old rotten furniture, ledgers, shoes and a slate urinal — were also found in the street that now looks more like a corridor. The arches had chambers built into them on several floors for storage for the shops at the top of the bridge, BBC reports.

The National Library of Scotland was built on top of the bridge, which was constructed on Libberton Wynd’s foundations that are still seen in the corridor dubbed by the librarians as The Void.