Whereas the Roman aqueducts now are only a historical curiosity, the Iranian system is still in use after 3,000 years and has continually been expanded. There are some 22,000 qanat units in Iran, comprising more than....

170,000 miles of underground channels. The system supplies 75 percent of all the water used in that country, providing water not only for irrigation but also for house-hold consumption. Until recently (before the building of the Karaj Dam) the million inhabitants of the city of Tehran depended on a qanat system tapping the foothills of the Elburz Mountains for their entire water supply.
Discoveries of underground conduits in a number of ancient Roman sites led some modern archaeologists to suppose the Romans had invented the qanat system. Written records and recent excavations leave no doubt, however, that ancient Iran (Persia) was its actual birthplace. As early as the seventh century B.C. the Assyrian king Sargon II reported that during a campaign in Persia he had found an underground system for tapping water in operation near Lake Urmia. His son, King Sennacherib, applied the "secret" of using underground conduits in building an irrigation system around Nineveh, and he constructed a qanat on the Persian model to supply water for the city of Arbela. Egyptian inscriptions disclose that the Persians donated the idea to Egypt after Darius I conquered that country in 518 B.C. Scylax, a captain in Darius' navy, built a qanat that brought water to the oasis of Karg, apparently from the underground water table of the Nile River 100 miles away. Remnants of the qanat are still in operation. This contribution may well have been partly responsible for the Egyptians' friendliness to their conqueror and their bestowal of the title of Pharaoh on Darius.
References to qanat systems, known by various names, are fairly common in the literature of ancient and medieval times. The Greek historian Polybius in the second century B.C. described a qanat that had been built in an Iranian desert "during the Persian ascendancy." It had been constructed underground, he remarked, "at infinite toil and expense ... through a large tract of country" and brought water to the desert from sources that were mysterious to "the people who use the water now."
Qanats have been found throughout the regions that came within the cultural sphere of ancient Persia: in Pakistan, in Chinese oasis settlements of Turkistan, in southern areas of the U.S.S.R., in Iraq, Syria, Arabia and Yemen. During the periods of Roman and then Arabian domination the system spread westward to North Africa, Spain and Sicily. In the Sahara region a number of oasis settlements are irrigated by the qanat method, and some of the peoples still call the underground conduits "Persian works." In the Middle East several particularly interesting qanats constructed by Arab rulers of early medieval times have been excavated. In A.D. 728 the caliph of Damascus built a small qanat to supply water for a palace in the country. A century later the caliph Mutawakkil in Iraq likewise constructed a qanat system, presumably with the aid of Persian engineers, that brought water to his residence at Samarra from the upper Tigris River 300 miles away.
it seems that these Qanats..are linked..underground... 170,000 miles of them that connect many countries of the world...