What is an Artesian Spring or an Artesian Well?
In order to understand an artesian spring, or artesian well, we must first have an understanding of artesian aquifers, which are their source. Simply stated, an aquifer is an underground body of water. Unconfined aquifers lay directly below the water table and collects its water mainly from rainfall. A confined aquifer lays below an unconfined aquifer, separated by a layer of clay or rock which restricts flow between the two water bodies. By confining the aquifer, the pressure from the weight of the water, or hydrostatic pressure, is always looking for a point of release. As such, any hole created which leads to the confined aquifer will be seen as the easiest way to escape confinement. An artesian is a confined aquifer that has found a way for the groundwater to flow upwards due to such pressure.
An artesian well is a man-made confinement tapped into an artesian aquifer which receives water without the need for pumping. Typically, artesian wells are cool water since the hydrostatic pressure is simply caused by gravity; for example, the inflow is a result of the melting of snow-capped mountains. However, an artesian spring known as a hot spring is often pressurized by heat from the earth's core. Extreme pressure of this sort may lead to geysers; springs which actually shoot out of the earth. Spring water is usually quite pure due to the natural filtration system of the earth: the water is cleansed by being forced through multitudes of pebbles, sand and limestone, clearing away debris and pollutants.
The largest and deepest artesian basin, or well, in the world is in Australia, aptly named the Great Artesian Basin. The aquifer lays under almost 25% of the continent, and is estimated to contain 64, 900 cubic kilometers of groundwater. That is almost 12 times the volume of all the Great Lakes combined!
A Frenchman, Artois, is credited for the naming of artesian wells and France still has an impressive system of artesian wells throughout the country. For example, Grenelle, near Paris, is typically sited as having the most "remarkable" artesian well, simply for being one of the first to descend to "remarkable" depths. At an impressive 1798 ft., the artesian well at Grenelle was the deepest of its time. To date, however, there are many wells reaching deeper than 2000 ft. to find the aquifer beneath.
In deserts such as the Sahara, the land and people rely on artesian springs almost solely for water supplies; these are popularly known as oases in such cases. Some of the more famous artesian springs reside at Yellowstone National Park. The largest geyser field in the world, Yellowstone features approximately 500 of the pressurized artesian springs.