On Friday, 21 October 1966, at 09:15, colliery waste tip number 7 (a "slag heap", containing unwanted rock from the local coal mine) slid down Merthyr Mountain. As it collapsed it destroyed two cottages and went on to demolish virtually all of Pantglas Junior School and part of the separate adjacent senior school.
In total 144 people were killed, 116 of whom were children mostly between the ages of 7 and 10.
Lord Robens of Woldingham, chairman of the National Coal Board, did not rush to the scene; he went instead to accept an appointment as chancellor of the University of Surrey. Subsequently he misrepresented the cause of the slide to the community and falsely claimed that nothing could have been done to prevent it.
At the Tribunal of Inquiry into the Aberfan Disaster, the National Coal Board was found responsible for the disaster, due to "ignorance, ineptitude and a failure of communication". The collapse was found to have been caused by a build up of water in the pile – when a small rotational slip occurred, the saturated, fine material of the tip liquefied, and it flowed down the mountain. The tip had been sighted on a known stream, which was shown on earlier ordanance survey maps, in 1958 and had suffered minor slips several times before. Its instability was known both to colliery management and tip workers but very little was done about it. The Merthyr Tydfil Borough Council and National Union of Mineworkers were cleared of any wrongdoing.
The NCB was ordered to pay compensation to the families: £500 per child. In a controversial move this payment was reduced by the amount that a publicly subscribed disaster fund paid to families. After lengthy appeals, part of the fund was used to make the remainder of the waste tip safe and the Coal Board avoided the costs of completely doing the job from their own resources but were later made to pay back this money into the fund.
Merthyr Vale Colliery was closed in 1989.