A huge spoil heap above a little village near Merthyr Tydfil in South Wales suddenly gave way, killing 116 children and 28 adults.
That was 50 years ago on Friday. As the world struggled to comprehend the disaster, the National Coal Board was criticised for “ignorance, ineptitude and a failure of communications” for not being alert to the fact that the tip could slip.
Most of the victims were children and teachers, suddenly suffocated in their classrooms. The Queen and then Prime Minister Harold Wilson were among those who visited the scene.
Today Aberfan - and indeed communities across the UK - will fall silent in memory of the moment when 150,000 tonnes of coal slurry swamped a row of houses, a junior school and part of a senior school.
Prime Minister Theresa May paid tribute to the victims and backed calls for a national moment of silence.
And Welsh First Minister Carwyn Jones said: “The disaster in Aberfan was one of the darkest days in modern Welsh history and we remember the adults and school children who lost their lives. We also think about the survivors, those who lost loved ones and the people who answered the call to search and care for survivors, and recover those who had perished.
“Individuals, families and the community have been profoundly affected by the disaster.
“Half a century after, it is fitting that the country as a whole comes together, with respect and compassion, to remember.”
On Friday, the Welsh Assembly will fly its flags at half mast as a mark of respect to those left devastated by the disaster and at 9.15am - the time of the disaster - people will observe a minute’s silence.
The facts of the tragedy are shockingly brutal. The disaster struck on Friday, October 21, 1966, when tip number seven collapsed and struck Pantglas Junior School, part of the senior school and a row of houses. It also fractured two water mains in the process.
The force from 150,000 tonnes of coal slurry was immense. A whole class of 34 juniors were among those who perished. But five children were miraculously dug out alive after they had been shielded from the brunt of impact by dinner lady Nansi Williams.
The horrendous aftermath was all the more painful to deal with given that there had been previous concerns from villagers.
A year before the tragedy unfolded, two mothers had presented a petition to Pantglas headmistress Ann Jennings about flooding - which she then passed on to the local council.
And in 1964, a local councillor - Gwyneth Williams - had said that if tip were to move suddenly it could threaten the whole school. However, the warnings were ignored - and the fight for justice afterwards left many in the village feeling even more angry.
Despite an in-depth tribunal, which cited the disaster had been caused by a “bungling ineptitude” by the Coal Board, no-one was ever punished. And insult to injury was to follow when a protracted row over the cost of removing Tip Number Seven saw scared villagers, keen for the tragedy not to happen again, left with little choice but to pay out £150,000 from a disaster fund .
The money was repaid in 1997. And the Welsh government went one step further 10 years on by donating £1.5m to the Aberfan Memorial Charity.
The anniversary should be marked by a nationwide moment of silence, Theresa May has said.
Gerald Jones, Labour MP for Merthyr Tydfil and Rhymney, asked Mrs May if she agreed that the nation should join members of his constituency to mark the tragedy.
Mrs May said what happened at Aberfan was a “terrible tragedy” as she agreed the nation should mark the occasion and remember those who died. She said: “I know that the Secretary of State for Wales (Alun Cairns) will be attending the particular memorial that will be taking place in your constituency on Friday. I think it is appropriate that we all mark and show our respect.”