For many years, millions of cubic metres of excavated mining debris from the colliery were deposited on the side of Mynydd Merthyr, directly above the village of Aberfan on the opposite side of the valley. Huge piles, or tips, of loose rock and mining spoil had been built up over a layer of highly porous sandstone that contained numerous underground springs, and several tips had been built up directly over these springs. Although local authorities had raised specific concerns in 1963 about spoil being tipped on the mountain above the village primary school, these were largely ignored by the National Coal Board's area management.