George Insole and his son James Harvey Insole purchased the Cymmer Colliery in 1844, sinking the No. 1 Pit three years later. Between 1852 and 1855, HM Inspector of Mines Herbert Francis Mackworth inspected the colliery twice and wrote letters to Insole recommending safety improvements, particularly to the underground ventilation system and the use of safety lamps. The warnings went unheeded. When the Crimean War created additional demand for coal in 1855, Insole doubled the number of colliers working the Old Pit and expanded the mine area by more than a third. Production intensified. Ventilation did not improve. Naked flames continued to burn underground where gas accumulated in the dark.

The explosion tore through the Old Pit shortly after the morning shift began. Defective ventilation had allowed firedamp to accumulate, and the naked flames that miners carried for light provided the spark. The blast killed 114 men and boys instantly or trapped them beyond rescue. Above ground, the colliery whistle brought families running to the pit head. Thirty-five women became widows that morning. Ninety-two children lost their fathers. The dependents of the dead had no legal right to compensation. Their only recourse was public charity and what the source article calls 'the final humiliation' of seeking poor relief from the parish.

At the coroner's inquest, Mines Inspector Mackworth testified that the explosion arose from 'the persons in charge of the pit neglecting the commonest precautions for the safety of the men.' The jury charged the mine manager Jabez Thomas and four officials with manslaughter. But James Insole, the owner who had ignored the inspector's warnings and driven the expansion that made the disaster inevitable, deflected responsibility onto his subordinates. The subsequent criminal proceedings exonerated all five officials. The mining communities of the Rhondda were outraged but powerless. Coal owners in the area went further, combining to deny work to any collier who had given evidence against the mine officials at the inquest and trial.

Insole contributed 500 pounds to the Cymer Widows' and Orphans' Fund and paid for thirty graves. The gesture was modest against the scale of the suffering. But the disaster did force changes. The single-shaft Old Pit and New Pit were eventually linked to create a safer two-shaft arrangement with better ventilation. Legislation tightened rules on mine ventilation, safety lamps, the employment of children underground, and the qualifications required of mine officials. The reforms came too late for the 114 who died, but they marked the beginning of a long, grudging acknowledgment that miners' lives had value beyond the coal they extracted. In 2015, forty schoolchildren walked seventeen miles from the former pit site to Insole Court, each carrying a lump of coal from Porth. At the end, they exchanged the coal for a golden coin, a symbolic gesture that the community still remembers.