Heavy rain led to a build-up of water within the tip which caused it to suddenly slide downhill as a slurry, killing 116 children and 28 adults as it engulfed Pantglas Junior School and a row of houses.

The tip was the responsibility of the National Coal Board (NCB), and the subsequent inquiry placed the blame for the disaster on the organisation and nine named employees.

After three weeks of heavy rain the tip was saturated and approximately 140,000 cubic yards (110,000 m3) of spoil slipped down the side of the hill and onto the Pantglas area of the village.

The organisation's chairman, Lord Robens, was criticised for making misleading statements and for not providing clarity as to the NCB's knowledge of the presence of water springs on the hillside.

Many of the village's residents developed medical problems as a result of the disaster, and half the survivors have experienced post-traumatic stress disorder at some time in their lives.

When the Merthyr Vale Colliery was sunk on 23 August 1869 by John Nixon and partners, Aberfan consisted of two cottages and an inn frequented by local farmers and bargemen.

[23] Complaints had been made by residents to Merthyr Tydfil County Borough Council, who corresponded with the NCB between July 1963 and March 1964 on the topic of the "Danger from Coal Slurry being tipped at the rear of the Pantglas Schools".

[c] G. M. J. Williams, a consultant engineer who gave evidence at the subsequent tribunal, stated that the 9:15 am movement: took part of the saturated material past the point where liquefaction occurred.

They directed the early digging, knowing that unplanned excavation could lead to collapse of the spoil and the remnants of the buildings; they worked in organised groups under the control of their pit managers.

[52] In his autobiography, Robens stated that the decision for him not to go was because "the appearance of a layman at too early a stage inevitably distracts senior and essential people from the tasks upon which they should be exclusively concentrating".

[54] NCB officers covered for him when contacted by Cledwyn Hughes, the Secretary of State for Wales, falsely claiming that Robens was personally directing relief work.

[59] A makeshift mortuary was set up in the village's Bethania Chapel on 21 October and operated until 4 November, 250 yards (229 m) from the disaster site; members of the Glamorgan Constabulary force assisted with the identification and registration of the victims.

Four hundred embalmers volunteered to assist with the cleaning and dressing of the corpses; a contingent that flew over from Northern Ireland removed the seats of their plane to transport child-sized coffins.

[63] In the early afternoon light rain began falling, which became increasingly heavy; it caused further movement in the tip, which threatened the rescue work and raised the possibility that the area would have to be evacuated.

When asked about the responsibility of the NCB for the slide, he answered: I wouldn't have thought myself that anybody would know that there was a spring deep in the heart of a mountain, any more than I can tell you there is one under our feet where we are now.

One man who had lost his wife and two sons called out when he heard their names mentioned: "No, sir—buried alive by the National Coal Board"; one woman shouted that the NCB had "killed our children".

[76][g] Sitting alongside Lord Justice Davies on the inquiry were the civil engineer Harold Harding and Vernon Lawrence, the former Clerk to the Monmouthshire County Council.

[81] Brian Gibbens, QC, the counsel for the National Union of Mineworkers (NUM), challenged Davies's evidence and stated that if the testament "is to be accepted as truthful and accurate in his recollection ... then he bears what must be one of the largest personal burdens of responsibility for the disaster".

The counsel for the families, Desmond Ackner, QC, strongly criticised Robens for making the statement about the spring, saying it was "a public scandal"; he added that "at no stage throughout this Inquiry has the National Coal Board taken the initiative to correct this sentence".

"[85] They added that the "legal liability of the National Coal Board to pay compensation for the personal injuries (fatal or otherwise) and damage to property is incontestable and uncontested".

These included years of rigid and unrealistic disregard for the importance of the safety of the above-ground tips (as opposed to dangers within the mines);[100] a flawed decision-making process which ignored or minimised the likelihood and the scale of the emergent danger; a dismissive attitude toward the complaints from Aberfan residents, discounting the validity of their concerns;[101] and an incomplete and inadequate response to conditions which caused those complaints.

[115] During a parliamentary debate on the disaster, Margaret Thatcher—then the opposition spokeswoman on power—raised the situation of one witness, criticised by the inquiry, who had subsequently been promoted to a board-level position at the NCB by the time the report was published.

[116][117] In 2000 Iain McLean, a professor of politics, and Martin Johnes, a research fellow in Welsh history, undertook a study of the Aberfan disaster and its repercussions; their work included government papers released in 1997 under the thirty-year rule.

All messages of support to him were catalogued by the NCB, and copies of some were leaked to the press;[119] the manoeuvring led to criticism in an editorial in The Guardian, which stated that "the Coal Board's behaviour has ... been rather unseemly in the circumstances".

[126][127][k] No specific aims for the fund had been outlined by the mayor until it was put on a firm legal footing under the auspices of a permanent committee with clear local representation.

[145]The residents of Aberfan petitioned George Thomas, who had succeeded Hughes as Secretary of State for Wales in April 1968, for the tips to be removed; they entered the Welsh Office and left a small pile of coal slurry on the table in front of him to make their point; Thomas later stated the tips "constitute a psychological, emotional danger" to the people of Aberfan.

[154] In May 1970 Barbara Castle, the Secretary of State for Employment and Productivity, appointed Robens to chair the Committee on Health and Safety at Work, to review legislation in the area and recommend the provisions that should be made for workers and the general public.

[155][156] In addition to news and historical coverage, the Aberfan disaster and its aftermath have been described in books, including histories of what happened, personal memoirs from those involved and collections of poetry, in music, song and on screen.

[173] In 1997 Ron Davies, the Secretary of State for Wales in the incoming Labour government, repaid to the disaster fund the £150,000 that it had been induced to contribute towards the cost of tip removal.

[179] In October 2016, the fiftieth anniversary of the disaster, memorial events took place at the garden and at the cemetery; the Prince of Wales represented the Queen, and government ministers were present to pay tribute.