"n the early part of the first millennium B.C., Persians started constructing elaborate tunnel systems called qanats for extracting groundwater in the dry mountain basins of present-day Iran (see figure 1). Qanat tunnels were hand-dug, just large enough to fit the person doing the digging. Along the length of a qanat, which can be several kilometers, vertical shafts were sunk at intervals of 20 to 30 meters to remove excavated material and to provide ventilation and access for repairs. The main qanat tunnel sloped gently down from pre-mountainous alluvial fans to an outlet at a village. From there, canals would distribute water to fields for irrigation. These amazing structures allowed Persian farmers to succeed despite long dry periods when there was no surface water to be had. Many qanats are still in use stretching from China on the east to Morocco on the west, and even to the Americas."
"Most rivers in Iran are seasonal and have traditionally not been able to supply the needs of urban settlements. Major rivers like the Arvand, Aras, Zayandeh, Sefid and Atrak were few and far between in the vast lands of Persian antiquity.
With the growth of urban settlements during the ages, locally dug deep wells (up to 100 meters deep) could no longer keep up with the demand, leading to the systematic digging of a specialized network of canals known as Qanat."
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