 As with any heavy industry , the possibility of serious injury or death was an everyday risk for the mine workers of the Rhondda Valley . The most notorious form of colliery disaster was the gas explosion , caused by either a buildup of methane gas or coal dust . As the mines became deeper and ventilation become more difficult to control the risk increased . The worst single incident in the Rhondda was the 1867 Ferndale disaster in which an explosion saw the loss of 178 lives . However , the major disasters only accounted for roughly 20 % of overall fatalities , with individual accidents accounting for the bulk of deaths . The list below shows mining disasters which saw the loss of five or more lives during a single incident . 
