valleys that had previously been sparsely inhabited suddenly increased in population. The Rhondda valley grew from less than a thousand people in 1851 to more than 150,000 in 1911. Between 1881 and 1911, Glamorgan became the most industrialised part of Wales and saw inward migration of more than 330,000 people from elsewhere in Wales, neighbouring parts of England and further afield. The coal mines were employing 250,000 men by 1921, but this was the peak and in subsequent decades the overseas market began to shrink. By 1930, employment in the mines was half of that in 1920 as mechanisation increased leading to a net loss of 314,000 people between 1921 and 1935. Accidents and safety The South Wales Coalfield was notorious for the number of fatal accidents in the 19th century. Risca Black Vein accidents The Risca Black Vein colliery had many fatal accidents to the extent that it became known as the "death pit". The two biggest accidents at the Black Vein pit were in 1860, when 146 died, and in 1880 when 119 people died. Prince of Wales disaster The Prince of Wales colliery in Abercarn exploded in 1878 causing 268 deaths. Llanerch Colliery disaster The Llanerch Colliery in Abersychan exploded in 1890 causing 176 deaths. Albion Colliery disaster The Albion Colliery in Cilfynydd exploded in 1894 causing 276 deaths. In the 20th century there were numerous further fatal accidents. Universal Colliery disaster The Universal Colliery in Senghenydd had Britain's worst mining accident when it exploded in 1913 killing 431. Aberfan disaster In 1966, 116 children and 28 adults were killed in Aberfan when a coal spoil tip collapsed onto them. Gleision Colliery On 15 September 2011, seven miners were working a narrow seam at the Gleision Colliery drift mine in the Tawe Valley, near Swansea, when a sudden ingress of water filled the passage in which they were working. Three miners were immediately able to escape to the surface. Despite extensive efforts to rescue the remaining miners, on 16 September South Wales Police confirmed that all four had died. Miners' health Coal mining in the South Wales Coalfield was a dangerous occupation with lifelong health implications. Between 1849 and 1853, miners over the age of 25 in the Merthyr Tydfil district were found to have a life expectancy of around 20 years lower than in other mining areas of England and Wales. In the 1930s, mine owners