Coal mining is the process of extracting coal from the ground. Coal is valued for its energy content and since the 1880s, has been widely used to generate electricity. Steel and cement industries use coal as a fuel for extraction of iron from iron ore and for cement production. In the United Kingdom and South Africa, a coal mine and its structures are a colliery, a coal mine - a pit, and the above-ground structures - a pit head. In Australia, 'colliery' generally refers to an underground coal mine. In the United States, 'colliery' has been used to describe a coal mine operation but nowadays the word is not commonly used. Coal mining has had many developments over the recent years, from the early days of men tunneling, digging, and manually extracting the coal on carts to large open cut and long wall mines. Mining at this scale requires the use of draglines, trucks, conveyors, hydraulic jacks and shearers. Small-scale mining of surface deposits dates back thousands of years. For example, in Roman Britain, the Romans were exploiting most of the major coalfields by the late 2nd century AD. The Industrial Revolution, which began in Britain in the 18th century and later spread to continental Europe and North America, was based on the availability of coal to power steam engines. International trade expanded rapidly when coal-fed steam engines were built for the railways and steamships. Until the late nineteenth-century coal was mined underground using a pick and shovel, and children were often employed underground in dangerous conditions. Coal-cutting machines were introduced in the 1880s. By 1912, surface mining was conducted with steam shovels designed for coal mining. The most economical method of coal extraction from coal seams depends on the depth and quality of the seams, and the geology and environmental factors. Coal mining processes are differentiated by whether they operate on the surface or underground. Many coals extracted from both surface and underground mines require washing in a coal preparation plant. Technical and economic feasibility are evaluated based on the following: regional geological conditions; overburden characteristics; coal seam continuity, thickness, structure, quality, and depth; strength of materials above and below the seam for roof and floor conditions; topography (especially altitude and slope); climate; land ownership as it affects the availability of land for mining and access; surface drainage patterns; groundwater conditions; availability of labor and materials; coal purchaser requirements in terms of tonnage, quality, and destination; and capital investment requirements.