Qanat

Qanats are a series of shafts connected by sloping tunnels that channel water underground over great distances. Dug deep into hills and mountains to take advantage of the higher water table, water is directed out into distribution canals. Being underground, little water is lost due to evaporation and so qanats provide a clean water supply relatively safe from natural disaster and sabotage. Following their development during the early 1st millennium BC, qanats spread rapidly across Africa and Asia. An overly-steep one would let the water flow too fast, draining the source and eroding the tunnel; if it were too shallow the water would not flow at all. Successful development of a qanat network required skilled artisans to locate potential water sources, plan routes, dig passageways to the right gradient and then keep them clear.