On October 21, 1966, 150,000 tonnes of coal waste from a colliery spoil tip collapsed into the village of Aberfan, engulfing Pantglas Junior School and neighbouring houses.
116 children and 28 adults were killed.
The tragedy has been retold for Netflix’s drama The Crown, which tells the story of the Queen’s visit to the village in the days that followed.
Where is Aberfan?
Aberfan is a former coal mining village in the south Wales Taff Valley near Merthyr Tydfil.
Welsh historian Peter Stead described the disaster as one that “takes us to the heart of the Welsh experience - coal.”
Coal mines dominated the south Wales Valleys during that period, with 250 collieries in Wales in 1947 according to the Northern Mine Research Society.
What happened on the day of the disaster?
240 children and nine teachers at Pantglas Junior School were waiting for lessons to begin on their last day of term.
But just minutes into the school day at around 9.15am, their school collapsed around them.
Among the first to react to the slide was Aberfan resident David Evans, who called emergency services after his neighbour said that she had heard a house collapse on Moy Road.
The colliery spoil tip that overlooked the village had collapsed, sliding down the mountainside and submerging the school and around 20 nearby houses.
A tribunal was held in 1967 to determine the cause of the slide that led to so many deaths. It found there was a rise in water pressure in the ground immediately beneath the colliery spoil tip.
A lack of action from the National Coal Board led to this water being released, triggering a slide that caused a 'mud run of tip rubbish,' according to investigators.
What happened in the aftermath?
Bodies were recovered from the rubble in the days after the disaster by emergency services, rescue teams, tip workers and local residents.
Makeshift mortuaries were opened in local chapels where fathers came to identify their children.
The villagers of Aberfan held a mass funeral six days after the disaster.
A memorial fund was set up on the day of the disaster, receiving nearly 90,000 contributions, but a forced payment of £100,000 was taken from the fund to clear the slide. It was not until 1997 that this money was given back to the fund.
A tribunal into the disaster found that The National Coal Board ignored repeated warnings that the coal slurry would not be able to withstand a period of heavy rain during the winter, and was likely to be a danger to Pantglas School.
The tribunal report gave an account of the events leading up to the disaster, including details about a prior slide of one of the tips in 1963.
Yet material continued to be tipped in the same way, except for tailings, with no inspection of the stability.
A witness stated that during the four months before the disaster the toe of Tip 7 moved downwards by around 20-30 feet, according to the tribunal report.
Aberfan residents and tip workers were also said to have expressed concern about a further slide that would eventually submerge the school and houses below.
The report described the attitude of the National Coal Board towards the 1963 slide, which it dismissed as a tailings “run” from the tip, as “insouciance”.
During the closing speech for the Board, the counsel admitted, “It need not have happened and should not have happened if proper site investigations had been carried out beforehand.”
The tribunal found the National Coal Board was to blame for the disaster.
The tragedy gave the previously little-known Welsh village national fame.
Prince Philip travelled to Aberfan days after the disaster to see the damage. The Queen later arrived and met with parents and children whose family members had been killed.
What is Aberfan like now?
A memorial garden has now replaced the space in which the school stood more than 53 years ago. The remnants of the coal tip can no longer be seen.
But the events of that day live on in the memories of survivors and the families of those who died.
Philip Thomas was one of only three schoolchildren to survive in one class of 32 at Pantglas Junior School.
"We could hear a rumbling noise, a bulldozer or a train or something rumbling, then it was like someone was throwing stones from the canal bank.
"We just run. But whatever hit me then, hit me on the head, and then I went down with my hands on the back of my head.
Philip and Robert were submerged by rubble. Robert did not survive.
Philip suffered life-affecting injuries including the loss of three fingers from his right hand - though memory loss brought on by the accident means Philip can't remember ever having them.
He said moving away from the area as an adult stopped him from having to recall the disaster whenever people asked where he was from.
In the aftermath, a group of young mothers came together to form a support group. The group grew to have sixty members and a waiting list. More than 50 years on, the Aberfan Young Vives’ Club continues to meet.
A male voice choir was also formed as a way of coping with the tragedy.
The Ynysowen Male Voice Choir, some of whose members lost children in the slide, has performed across Europe and holds concerts to raise money for charity.