On the morning of Tuesday 15th July 1856 at the 'Old Pit' in Cymmer, there occurred as stated in the Mines Inspector's report, 'The most fearful and destructive explosion, resulting in a sacrifice of human life unparalleled in the history of Britain at that time'. At 6.00 a.m. one hundred and sixty men and boys descended the shaft to begin their shift and were on their way to their working places when the explosion took place. The ferocity of the explosion led rescuers to believe that all lives below ground would be lost. However some miners had only descended a short way into the pit and were able to make their way back to the shaft and safety. Rescuers took measures to clear the mine of afterdamp, and descended into the mine some three hours after the initial explosion took place. By that Tuesday evening some 112 bodies had been brought to the surface of the mine, another was recovered on the Wednesday and one severe burns victim died on the Thursday.

At the pithead relatives and friends searched to identify victims, few of whom were not badly burnt, dying instead of suffocation caused by the afterdamp. Temporary mortuaries were set up in the Colliery's carpenters shop and also Cymmer Independent Chapel.

The scale of the disaster devastated the small communities of Cymmer and surrounding areas, with no household left untouched and virtually all working age men and boys having perished. One household alone saw a father and his three sons aged 10, 13, and 16 all perish. Thirty graves were opened at Cymmer Independent Chapel's graveyard, and a mass internment of victims took place on Thursday 17th July.

The first inquest into the disaster was held at the Ty Newydd Hotel, Porth on the 16th July, which was subsequently adjourned until Monday 27th July at the Butchers Arms in Pontypridd. The inquest lasted 13 days and twenty-nine witnesses were called. At the inquest it became clear that safety precautions at the colliery were woefully inadequate, and mines safety regulations were ignored. It was seen that the mine was poorly ventilated, and that pockets of gas were an everyday feature of life below ground, additionally the carrying of naked flames in the mine was commonplace. The jury returned a verdict of manslaughter on the colliery manager, Jabez Thomas, and four senior officials at the mine. This verdict was overturned at the Glamorgan assizes at Swansea, the following March, leading to a great deal of resentment among colliery workers.