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A well is a hole that is dug into the Earth to get water (or crude oil or natural gas). A qanat is an ancient complex water well system used in the Middle East. Wells can be as simple as a hole that a bucket on a rope can be lowered into, or very complex with pipes and high-powered pumps to get the water out. Most cities that are not close to fresh water lakes or rivers get their water from wells.
A well only works if underneath there is an aquifer which feeds it.