The worst pit-disaster of the 20th century occurred at the Universal, Senghennydd on 14 October 1913.
More than 400 men were trapped underground by an explosion and fire. It ripped through the underground tunnels just after 8am - two hours into the morning shift. They say the explosion was heard 11 miles away in Cardiff. Rescuers battled for days to recover the wounded and the dead. The first funeral was held three days later, but it was not until the middle of November that all 300 bodies were recovered from the Mafeking, Pretoria and Kimberly mines. 440 miners, men and boys, lost their lives.
As bad as the 1901 disaster had been it was to be overshadowed by the biggest mining disaster in Great Britain, which occurred at this colliery on Tuesday 14th of October 1913, killing 439 miners.
It was 8 o'clock in the morning with a workforce of 950 men underground when there was a massive explosion, the force of which sent the cage in the Lancaster pit rocketing up the shaft and crashing into the pithead gear. The force of the blast smashed the wooden platform on which the banksman John Mogridge was standing; he was decapitated by a large splinter of timber.
There had been little damage to York pit, so the manager and some other men slowly descended this shaft. When they reached about 530 yards down some girders blocking their progress halted their descent, but by shouting down the shaft to the nine-foot level they were heartened by the replies that some of the men were safe.
It was probably started by a firedamp (methane) explosion, itself possibly ignited by electric sparking from equipment, such as electric bell signaling gear. The initial firedamp explosion disturbed coal dust present on the floor, raising a cloud that then ignited in its turn. The shock wave ahead of the explosion raised yet more coal dust, so that the conflagration was effectively self-fueling. Those miners not killed immediately by the fire and explosion would have died quickly from afterdamp, the noxious gases formed by combustion. These include lethal quantities of carbon monoxide, which kills very quickly by combining preferentially with haemoglobin in the blood. The victims are suffocated by lack of oxygen.