This remarkable complex of tunnels, 8km north of Kashan, originally grew up around a freshwater spring, credited with supplying delicious, crystal-clear water. Only part of the tunnel system is open to visitors today, and those parts are often subject to flooding (note the two-colour tone of the walls showing the flood level), but even a quick descent to the first level gives an idea of the complexity of this ancient engineering project.
The original purpose of the tunnels and chambers that were dug around the original well may have been to provide a respite from the desert's summer heat but they also appear to have had a further function in allowing inhabitants to move from one part of their town to another without having to encounter potential enemies. Considered a masterpiece of Sassanian architecture, they were constructed on three levels between 4m and 18m below ground level and included a number of ingenious devices to trap and ambush hostile intruders, such as curving corridors and disguised pits covered with stones. The tunnels, which were put to good use during the Mongol invasion in the 13th century, worked particularly well as an emergency shelter because there were several entrances to the underground chambers, some of which surfaced within the town's houses. Ventilation shafts allowed for prolonged residence underground and fresh water was assured by the spring. The tunnels were eventually abandoned in the 1920s and found by accident when someone dug a well in their house and stumbled through to the labyrinth of chambers below.