A drilling rig is an integrated system that drills wells, such as oil or water wells, or holes for piling and other construction purposes, into the earth's subsurface. Drilling rigs can be massive structures housing equipment used to drill water wells, oil wells, or natural gas extraction wells, or they can be small enough to be moved manually by one person and such are called augers. Drilling rigs can sample subsurface mineral deposits, test rock, soil and groundwater physical properties, and also can be used to install sub-surface fabrications, such as underground utilities, instrumentation, tunnels or wells. Drilling rigs can be mobile equipment mounted on trucks, tracks or trailers, or more permanent land or marine-based structures (such as oil platforms, commonly called 'offshore oil rigs' even if they don't contain a drilling rig). The term "rig" therefore generally refers to the complex equipment that is used to penetrate the surface of the Earth's crust. Small to medium-sized drilling rigs are mobile, such as those used in mineral exploration drilling, blast-hole, water wells and environmental investigations. Larger rigs are capable of drilling through thousands of metres of the Earth's crust, using large "mud pumps" to circulate drilling mud (slurry) through the drill bit and up the casing annulus, for cooling and removing the "cuttings" while a well is drilled. Hoists in the rig can lift hundreds of tons of pipe. Other equipment can force acid or sand into reservoirs to facilitate extraction of the oil or natural gas; and in remote locations there can be permanent living accommodation and catering for crews (which may be more than a hundred). Marine rigs may operate thousands of miles distant from the supply base with infrequent crew rotation or cycle. History Until internal combustion engines were developed in the late 19th century, the main method for drilling rock was muscle power of man or animal. The technique of oil drilling through percussion or rotary drilling has its origins dating back to the ancient Chinese Han Dynasty in 100 BC, where percussion drilling was used to extract natural gas in the Sichuan province. Early oil and gas drilling methods were seemingly primitive as it required several technical skills. The skills involved the availability of heavy iron bits and long bamboo poles, the manufacturing of long and sturdy cables woven from bamboo fiber, and levers. Heavy iron bits were attached to long bamboo cables suspended from