Who or what is the Big Grey Man of Ben Macdui?

It is almost 100 years since Prof John Norman Collie told of encountering a terrifying presence on a Scottish mountain that would become known as the Big Grey Man.

But what happened on Ben Macdui, and could an atmospheric phenomenon occasionally seen in the hills - including in the past few months - offer an explanation?

Who was Prof Collie?

From Alderley Edge, near Manchester, he was a highly respected 19th Century scientist, explorer and mountaineer.

He is renowned for his work with mountain guide John Mackenzie, exploring and mapping Skye's Black Cuillin mountains, and their contribution to mountaineering is recognised by a statue in Glen Sligachan.

What happened on Ben Macdui?

A view of Ben Macdui and Derry Cairngorm from Beinn Mheadhoin

In 1925, Collie was invited to speak at the Cairngorm Club in Aberdeen.

He stunned his audience with an account of a climb he made more than 30 years earlier of Ben Macdui - Britain's second-highest mountain.

He said while walking from the summit through mist he heard a sound of crunching footsteps behind him.

Collie looked around but could see nothing. The footsteps continued - they were following him.

He said he was terrified and ran for miles to escape the sinister presence.

Newspapers picked up the story and the legend of the Big Grey Man of Ben Macdui was born.

Have there been other encounters?

A M Kellas, a veteran of Himalayan ascents, is said to have later told Collie of a similar terrifying experience he had on Ben Macdui.

Mountaineer Alexander Tewnion claimed to have seen the Big Grey Man during a mist-shrouded ascent of Ben Macdui in 1943.

He described seeing a shape loom up and then charge at him.

Tewnion happened to be carrying a revolver and fired at the figure before running away.

And way back in the 1790s, poet James Hogg wrote of confronting a shadowy figure in the Cairngorms while tending sheep.

He took fright and ran, but returned the following day to see the same thing again.

But Hogg discovered during this second encounter the figure moved as he moved.

It is believed Hogg had seen the atmospheric phenomenon known as a Brocken Spectre.

What is a brocken spectre?

It is a large shadow cast onto cloud or mist. This example with a fog bow arching overhead was pictured by Derek Petrie in February on Beinn A'Ghlo

The shadow is an optical illusion that can give the impression of a gigantic figure. John Garbett took this photo on Ben Vorlich in February.

Huw Owen's picture of a brocken spectre was taken in January while on Schiehallion.

But what about the crunching footsteps?

Chris Townsend is an experience climber of Ben Macdui

Brocken spectres are silent, and do not explain the sounds Collie said he heard.

Chris Townsend has climbed Ben Macdui hundreds of times in all weathers.

The outdoor writer, photographer and long-distance walker said: "My guess is that the sounds and occasional sightings are to do with disorientation in mist or darkness amplifying the volume of sounds and the size of objects."

Townsend has another theory to the legend.

"Norman Collie did have a reputation as a practical joker and this idea for the Big Grey Man does appear in a few places.

"His status as a top mountaineer and a scientist certainly raised the profile of the Big Grey Man and led to others saying they had heard or felt something."

Don't rule out the supernatural, says writer

Award-winning Danny Robins hosts the BBC's Uncanny podcast and TV show, and wrote West End play 2:22 A Ghost Story.

He said explanations could include physical and mental effects of being in an extreme environment, and contagion - one person's fears spreading to others.

Robins said there was another possibility - ghosts - adding that Scotland's hills are no stranger to tragedies and other unexplained incidents.

He said: "One of our Uncanny cases was Luibeilt Lodge, a mountain bothy people have said is haunted.

"People unconnected to each other and without prior knowledge have told of similar experiences there.

"That intrigues me."