What caused Aberfan disaster?

The local colliery run by the National Coal Board had instructed that all waste from the pit was to be tipped on land above the village. Under just one of the tips was a natural spring that led to the River Taff. On 21-Oct 1966. Despite warnings people responsible did not stop tipping and did not undertake a risk assessment. At 09;15 the shale and waste became a slurry and 1 million cubic feet of this slurry ran down the mountain engulfing the local school and 18 houses.