A glint of light in the darkness of the unfolding Aberfan disaster led to a photographer taking what became the iconic image of the tragedy.
Mel Parry, an apprentice newspaper photographer in Merthyr Tydfil, snapped a shot of the rescue scene which contained a policeman carrying schoolgirl Susan Robertson (nee Maybank) after she was pulled from the wreckage of Pantglas Primary School.
The school and nearby houses were covered with waste from a coal tip which slid down Merthyr Mountain behind the south Wales village on 21 October 1966, killing 116 children and 28 adults.
When blown up and cropped, the image of Susan in the policeman's arms appeared on the front page of papers across the world by the next day.
Mr Parry, who was 18 at the time, had got on the bus in Aberfan to go to work in nearby Merthyr Tydfil when he heard there had been a coal waste slip at the school.
He said: "I got off the bus, saw it, rang the office and asked the chief photographer if he could bring some equipment down. As soon as he arrived, I just started taking pictures.
"The photograph that everybody's aware of I have no recollection of taking. It was, from what I'm led to understand, one of the first three that were ever taken of the site."
The only thing he remembers is seeing a glint of light through the camera.
"I had no idea what it was at the time but after seeing the actual photograph I can see it was the top of the policeman's helmet because that's the only thing that was glinting.
"I saw the photograph later in the evening when the paper came out. I didn't think anything of it, I didn't even think it was mine - I didn't find out until three days later," he said.
Mr Parry went on to win news category of the British Press Photographer of the Year in December that year, the youngest-ever recipient at the time.
It briefly brought him fame but it is not something he remembers with pleasure.
He said: "Personally I wish I'd never taken it, because I wish the disaster had never happened. I just happened to be one person in the right place at the right time. Six or seven years later I got out of photography altogether.
"It gave me 15 minutes of fame on the back of a disaster and that is something I would not wish on anybody."
'Chain of men'
For Susan Robertson, the photograph has made her the repeated focus of attention over the years at successive anniversaries
"It went worldwide and this is why every so many years, photographers and journalists always come looking for me," she said.
"I remember every moment from that morning. I was in the classroom and it all went black and there was a rumbling like thunder and this is when the tip hit the school.
The picture has kept Susan Robertson in the public eye
"I was buried and I heard voices above my head. I could hear the men, way above the window and they said there's a little girl here.
"They asked me to put my hand through the inkwell of the desk but it was too small so I put my finger through and this is when they dug down and pulled me up.
"I was carried through a chain of men through the hall, of miners I should imagine, and they passed me from this one rescuer through a window to the policeman."
Mrs Robertson said the reason she was crying in the photograph was because she had lost a shoe.
"My aunty Alice [pictured next to her] was saying, 'come on Susan, come on you're all right'. I was glad to have seen her, then my mother came along.
"When I came home from hospital I saw the picture in the paper, and said to my mother and father, 'look, that's me'."
The fact the picture became so famous was down to an observant darkroom assistant, Alun John.
He was working at the South Wales Echo in Cardiff the morning of the disaster when the roll of film taken by Mel Parry was brought in.
"When I got to the third frame, I could see that although it was of a general scene, there was obviously something quite important in the middle of the picture," he said.
"I pushed the enlarger up as high as I could and it still wasn't big enough so I turned the enlarger around and shone it onto the floor.
"In the middle of the picture, I could see this policeman with the child in his arms and the very, very upset lady next to him.
"I immediately made a print of just this small section, processed it and dried it very quickly and literally ran it outside to the picture editor - quite excited I was by this stage.
"I said 'I think there's possibly something more here than we thought in the first place'. He looked at it and went, 'Yeah, you're right' and he ran off to the main newsroom.
"Forty minutes later it was on the front page of the Echo and an hour later it had gone round the world."
Mr John compared the impact of the photograph to the image from the 11 September World Trade Center attacks of a plane hitting the second tower.
"If you think what an iconic image that was on the day, exactly the same happened to [the Aberfan picture]," he said.
"That picture became the defining image of the story. It was used on every front page of every national newspaper in this country, it was used in news bulletins, and news magazines overseas."