A borehole is the generalised term for any narrow shaft drilled in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes including the extraction of water or fluid (such as oil) or gases (such as natural gas or methane), as part of a geotechnical investigation or environmental site assessment, for mineral exploration, or as a pilot hole for installing piers or underground utilities. Boreholes used as water wells are described in more depth in that article. In the engineering and environmental consulting fields, the term is used to collectively describe all of the various types of holes drilled as part of a geotechnical investigation or environmental site assessment (a so-called Phase II ESA). This includes holes advanced to collect soil samples, water samples or rock cores, to advance insitu sampling equipment, or to install monitoring wells or piezometers. Samples collected from boreholes are often tested