1911 the population increased by 320,000. By 1913 the Welsh collieries were extracting 56.8 million long tons of coal (63.6 million short tons) a year, up from 8.5 million long tons (9.5 million short tons) in 1854; collieries in the region mined a fifth of all coal produced in the UK, and employed a fifth of its miners in the mid-nineteenth century. In 1913 Britain was responsible for 25 per cent of world coal production and 55 per cent of all world coal exports. The South Wales Coalfield produced the sought-after anthracite, bituminous and steam coals—the latter a grade between the two comprising a hard coal without the coking elements. Some of the region's coal seams contained high quantities of firedamp—a mixture of methane and hydrogen—and were therefore prone to explosions; firedamp rises into the higher points of workings, including cavities or, as at Senghenydd, when the seams were being mined in an upward gradient. An additional danger of firedamp is afterdamp, a poisonous mixture of gases left after an explosion, primarily constituted of carbon dioxide, carbon monoxide and nitrogen. The carbon monoxide combines with haemoglobin in the bloodstream to form carboxyhemoglobin, which prevents the blood cells carrying oxygen, and can therefore result in suffocation by lack of oxygen known as anoxia. If survivors from an explosion are not rescued quickly, they face the possibility of being killed by the gas. The presence of firedamp in South Wales's collieries contributed to a higher-than-average proportion of accidents: between 1880 and 1900 South Wales accounted for 18 per cent of Britain's miners, but 48 per cent of all UK mining deaths occurred in the region. As coal output from British collieries reached its peak in 1913 there was a correspondingly large number of accidents around this time. Senghenydd and the Universal Colliery Senghenydd— in Welsh—is situated at the northern end of the Aber Valley, approximately north-west of Caerphilly and north-west of Cardiff. When geological surveys for coal began in 1890 it was a farming hamlet of around 100 people. Coal was found, and sinking of the first mineshaft for Universal Colliery—which was owned and developed by William Lewis—began in 1891; the first coal was extracted in 1896. The colliery's two shafts were both deep, the downcast Lancaster and the upcast York. Development of the pit coincided with the Boer War, and sectors of the underground workings were named after key places in