“I try to rationalise it, I can’t. I have never experienced anything else like this in my life.”

My favourite ghost stories are the ones told by people who don’t believe ghosts exist. There’s nothing more unsettling than a hardened sceptic who’s witnessed something terrifying they cannot explain.

“There was a very strong sense of pure distilled evil coming from this figure.”

Ken is a geneticist. He spends his days studying genomes, trying to work out the causes of diseases. He’s used to rational scientific thought, but he’s describing to me an experience that, on paper, is impossible; a tale of poltergeist activity and apparitions that could have leapt off the pages of an MR James ghost story.

Ken became a student in Belfast, Northern Ireland, in the autumn of 1981, arriving in a city where the sectarian conflict known as the Troubles was still ongoing, providing a constant backdrop of bombings and attacks. His accommodation was an ugly tower block known as Alanbrooke Hall, sharing a room on the sixth floor: Room 6–11.

“It’s been ingrained in my mind for a long time, that number,” says Ken. He’s in his 60s now, wearing glasses that remind me of Michael Caine. He’s keen to emphasise how ordinary Room 6-11 was – “the last place in the world you’d expect anything to happen” – yet, on that winter night in 1981, as he settled down in bed, he turned to see a figure like a shadow come to life, the black silhouette of a man.

“I couldn’t see any features; it was just the blackest black I’ve ever seen.” As the figure advanced, Ken was struck by what he describes as a “force of nature”, like waves of energy, a sensation of total evil.

I can see Ken realises how ridiculous this might sound. Perhaps he’d have dismissed it as some strange hallucination, except it heralded a sequence of bizarre poltergeist activity experienced both by him and his roommate over several months; heavy desk drawers opening by themselves, items moving inside cupboards, and pages of books turned by an invisible hand. Most terrifyingly, one night when Ken was reading, knowing he was entirely alone on the sixth floor, everybody else having gone home to their families for the weekend, there was a savage pounding on the door, threatening to break it off its hinges. Using every ounce of courage to open it, he discovered the corridor was totally empty.

Eventually, Ken decided to talk to the students who’d lived in the room the previous year. This is where the story gets really weird, because they had also experienced poltergeist activity, with books actually flying at them! When it was finally time for Ken to move out, he decided not to tell the new occupants what had happened to him. A few weeks later, the new guys sought him out to describe their experiences – even more frightening than Ken’s!

So, what was it about Room 6-11 that terrified successive occupants? My inner psychotherapist wonders if the backdrop of the Troubles in Belfast helped stoke these feelings of fear and anxiety, yet the experiences these young science students described to each other feel utterly real and compelling. I’ll be trying to solve the mystery in my new BBC Sounds series Uncanny, as I throw the investigation open to the public, searching for anyone else who had similar experiences in Alanbrooke Hall. Will we finally discover the truth behind ‘the evil in Room 6-11’?