The Aberfan disaster, where 144 people lost their lives, still haunts the community at the heart of the tragedy.
At 9.13am on 21 October 1966, 150,000 tonnes of coal waste slid down a hillside, engulfing Pantglas Junior School and a number of neighbouring houses.
In Aberfan more than 200 children and nine teachers were waiting for their first lesson of the day to begin when a landslide of mud and debris flooded into the classroom - 116 children and 28 adults were killed.
It was one of the worst industrial disasters Britain has ever seen.
What happened in Aberfan that morning?
The school day had not long started when, witnesses later said, there was an ominous rumble in the distance.
A massive coal tip - a mountain of waste generated by the town's mines that employed 8,000 people - had collapsed, sending thousands of tonnes of slurry sliding into the village.
The avalanche of slurry buried the school and engulfed the 240 pupils and staff inside.
Dilys Pope, 10, would later describe what happened.
She said: "We heard a noise and we saw stuff flying about. The desks were falling over and the children were shouting and screaming."
The deputy head teacher, Mr Beynon, was found dead.
A rescuer said "he was clutching five children in his arms as if he had been protecting them".
How did the disaster happen?
There were 11 tips in the hills overlooking Aberfan. The one that collapsed was Tip Number 7; standing more than 111ft high, it was begun in 1958 and was estimated to contain 140,000 cubic yards of waste.
The mountain of spoil sat on underground springs; weeks of heavy rainfall had saturated the ground making the spoil heap unstable.
Members of the morning mining shift clocking on at 7.30am on 21 October after another night of heavy rain noticed there had some slippage in Number 7 tip.
At 9.15am, a swathe of Tip Number 7 broke away and began sliding down the hill towards the village.
What was engulfed by the avalanche of waste?
Moving at estimated speeds of 20mph, the wave of waste swept through the village at a height of between 20-30ft.
The slurry covered the primary school, some terraced houses and a local farm - and within minutes 116 children and 28 adults were dead.
Many local miners took part in the rescue effort and used shovels to get the debris clear.
In total, about 2,000 rescuers dug frantically, some for over 10 hours, in a desperate attempt to save lives - but only a handful of children survived.
If the coal tip had slipped just a few minutes earlier the children would not have been in their classrooms.
Just a few hours later, they would have broken up for half-term.
Who was to blame?
The Aberfan Disaster Tribunal was established within days of the incident. It was chaired by the Welsh judge and Privy Councillor Lord Justice Edmund Davies, who was born two miles from Aberfan.
The tribunal sat for 76 days, and heard from more than 130 witnesses. The National Coal Board - then in charge of Britain's mines - had disputed any claims that it was responsible for safety failings.
In its report on the disaster, the tribunal concluded: "The Aberfan disaster is a terrifying tale of bungling ineptitude by many men charged with tasks for which they were totally unfitted, of failure to heed clear warnings, and of total lack of direction from above.
"Not villains but decent men, led astray by foolishness or by ignorance or by both in combination, are responsible for what happened at Aberfan.”
It laid the blame squarely at the door of the National Coal Board, and certain individuals. The NCB paid out £500 each to the families of those who perished.
How is the disaster still marked?Schools across Wales held a minute of silence at 9.15am on the morning of 21 October to remember those lost.
As a way to educate the younger generation on the disaster, children at a school in Church Village, Pontpridd have been studying what happened that day in Aberfan.
The pupils have put together a theatrical performance, poetry, literature recitals, as well as a special exhibition at Canolfan Garth Olwg Centre in the village.
One Year 12 pupil said the tragedy had brought both the school and the local community together.
Another explained how important it was to "learn about local history".
Talking about the drama production, one boy said he felt "scared" acting in the role, thinking of the young children living through the experience but "not knowing what was happening".
A striking and poignant sculpture has also been unveiled at the Rhondda Heritage Museum in memory of those who lost their lives at Aberfan.
The sculpture by renowned Welsh artist Nathan Wyburn has been installed at the Coal Mining Experience at Rhondda Heritage Park.
The artwork, called 21.10.1966 144 9.13AM, is made from concrete and corten steel. It contains 144 clocks - the same number of adults and children who died when a colliery spoil tip collapsed into homes and Pantglas Junior School.
Each of the clocks are set to 9.13am - the exact time the disaster happened on October 21 1966.
Since the disaster, plans to replace "outdated laws" regulating the safety of coal tips were launched in June 2021.
The management of the coal tips have now been described as "no longer fit for purpose".
The proposals include the creation of a body that would be responsible for the safety of all disused coal tips.