400 hundred men were cut off in the west section of the Senghenydd colliery, near Caerphilly. The explosion was probably started by firedamp, possibly by electric sparking from equipment such as electric bell signalling gear. The initial explosion disturbed coal dust present on the floor, raising a cloud that then also ignited. The shock wave ahead of the explosion raised yet more coal dust, so that the explosion was effectively self-fuelling. 439 miners were killed, it was the worst mining accident in the United Kingdom.