The Aberfan disaster was the catastrophic collapse of a colliery spoil tip in the Welsh village of Aberfan, near Merthyr Tydfil.
It killed 116 children and 28 adults on 21 October 1966.
The collapse was caused by the build-up of water in the tip, which suddenly slid downhill as slurry.
More than 1.4 million cubic feet (40,000 cubic metres) covered the village in a matter of only a few minutes. The classrooms at Pantglas Junior School were immediately swamped. Children and their teachers died from either impact or suffocation.
Ironically, if the disaster had struck a few minutes earlier, the children would not have been in their classrooms, and if it had struck a few hours later, they would have left for their half-term holiday.
The ramifications were widespread but, of course, nothing could bring back those who had died.