the 1960s by the National Coal Board (for instance, Cynheidre Colliery No 1 shaft, at deep was sunk in 1954/6). Following the general collapse of the UK coal industry, most pits closed during the 1980s, with factors such as exhaustion of reserves and geological complexity adding to their problems. The last deep mine, at Tower Colliery on the north crop, ceased mining in January 2008. However, a few small licensed mines continue to work seams, mostly from outcrop, on the hillsides. Although some areas of the coalfield are effectively worked out, considerable reserves remain. However, the geological difficulties, which resulted in the closure of (for instance) Nantgarw colliery, make the cost of significant further extraction high. The coalfield experienced a late-stage development when opencast mining was commenced on a large scale, mostly on the gently-dipping north crop. In addition, old tips were reclaimed for their small coal content, which could be burned in power stations such as nearby Aberthaw. Most of the old sites have been filled and landscaped, but new operations continue. Following the Aberfan disaster of 1966, when a coal-tip slurry flow buried a school, mine-waste tips, which had been piled precariously on hilltops in many cases, were extensively regraded and reclaimed. This work continues. Landslipping of the steep valley slopes, and subsidence caused by the coal extraction, have also posed problems. Minor revival A subsidiary of Western Coal (which mainly operates in the British Columbia and West Virginia coalfields), Energybuild plc, worked a drift mine near the old Tower Colliery, the Aberpergwm Colliery, until production was suspended in July 2015. The mine produced high quality anthracite, and was sold primarily to Aberthaw power station and Port Talbot Steelworks and into the wholesale and retail sized coal markets. Energybuild also operates the Nant y Mynydd opencast coal site nearby. Sociology and demography of the South Wales Coalfield Population change As the mines and other industries rapidly expanded throughout the coalfield, nearby towns also expanded to meet the demand for labour. In the first half of the nineteenth century, the development of ironworks saw the population of Merthyr Tydfil, in the northern part of the coalfield, increase from 7,700 in 1801 to 49,794 in 1861, making it the largest town in Wales. As Cardiff and other ports in South Wales grew to meet the demands for exporting iron, steel and coal in the later part of the nineteenth century,