How did qanats carry groundwater from hillsides to towns in ancient Persia?

A qanat is an ancient water-supply system that functions by tapping into groundwater trapped within or beneath the upper reaches of alluvial fans and hillsides.12

To transport this water to towns and cities, engineers constructed a series of gently sloping tunnels driven directly into the hillsides. By maintaining a precise, slight downward gradient, the system utilized the force of gravity to convey the water over distances that could span several kilometers. Once the water reached the surface from these underground channels, it was typically distributed to nearby communities through open channels for domestic use and irrigation. This sustainable, gravity-fed technology was so effective that it remained a primary water source for cities like Tehran until the early 20th century.123