Up until 1966, the National Coal Board had allowed the excavations from the Aberfan mine to be piled up on the hill above the village. The total volume of this debris is unknown, but the estimated volume of just the portion that broke away on October 21, 1966 is in the vicinity of 150,000 cubic metres. Safety inspectors were typically more concerned with safety issues inside the mine than outside, but even so the NCB had been warned repeatedly over the years leading up to this disaster.
The warnings went unheard until that deadly Friday, when heavy rains contributed to the slide of the debris onto the town. The debris covered a farm, twenty houses along Moy St, Aberfan, and a large portion of Pantglas Junior School. The total death toll was 144 people, including 5 teachers at the school, and 115 students aged between 7 and 10 years old. It remains one of the worst mining disasters of the modern era.