This is the name in Persian for a water channel dug underground at a precisely calculated angle of slope to bring water from a source usually under mountains or hills down to the arid land of the plains where people live. In effect, an underground aqueduct. They consist of a tunnel barely high enough for a bending man to pass, with regular vertical shafts used both in the construction process and for regular maintenance. The earliest known qanats date to around 2,700BC, and thousands are still in use today. The building of a qanat was a highly specialised process, since the gradient sometimes had to be worked out over a length of up to 50-70 kilometres, although most are much shorter than that. The long straight lines of excavated circles of earth around the vertical shafts are visible from the air, and were often one of the only features visible on domestic flights over the south-east of Iran. They stretch across the desert landscape at regular intervals of about fifty metres. The main well at the head, artificially created from an underground spring, can be up to 300 metres underground. It was therefore expensive and difficult to build a qanat, and was often carried out as an investment by landlords, who could then sell the water. For the specialised labourers who dug and maintained tunnels and shafts, known as muqannis, it could be dangerous work - when tunnel roofs collapsed for example.

Since the water never comes to the surface until the end of the qanat there was obviously not much evaporation even in the summer months when temperatures in south-eastern Iran reach into the mid- and upper-40s centigrade.

This expertise was exported to several neighbouring countries, and I now know that the Turpan water system in Xinjiang - where the qanat is known as a karez - was probably introduced by Persian experts during Tang times. In this case, water originating in the Tian Shan Mountains was brought down to supply the needs of Silk Road caravans and travellers on the rim of the Taklaman Desert.

These photographs were taken in 1978 near Kerman, in south-eastern Iran: in the first, looking across the arid landscape the circular mole-like heaps of excavated earth, or miniature bomb craters, mark the opening of a vertical shaft and illustrate the direction of the qanat down to the village whose single-storey houses can be seen on the horizon; the second photo shows the opening of the qanat in the village at ground level, where the crystal-clear water could then be collected for household use, and switched through gates and dams into irrigation channels for agricultural use as needed.