An excavation made to extract water, oil, gas, brine, or other fluid substance from the earth. Wells are the source of about one quarter of the water supplies in the United States, all gas and oil, and most of the industrial brines and sulfur. Water wells are recorded in the earliest historic documents and probably originated during periods of drought when ancient humans attempted to reach water by digging a shallow excavation at the site of springs that had ceased to flow or in the dry channels of rivers. Later they learned to dig deeper for water where it did not issue at the land surface. The first wells were dug by hand, and some large deep ones were provided with elaborate ramps which enabled those drawing water to walk or even drive a donkey down to the water level. In the Near East for thousands of years wells (khanats) have been constructed by tunneling nearly horizontally into the outwash gravel to tap ground water which flows down the tunnel to its entrance. Horizontal wells are also used in Hawaii to skim fresh ground water from underlying salt ground water.