Aerial photographs of the Iranian city of Bam, which was destroyed in an earthquake last year that killed more than 26,000 people, have revealed important new archaeological sites.

One discovery dates from between 2,400BC and 2,600 BC, proving the city is centuries older than experts had thought. Another site, from medieval times, showed that the community then practiced religious and cultural tolerance but was threatened by marauding Turkic tribes and the Mongol invasion.

The history of the city rests on an astonishing network of qanats, huge underground irrigation channels, kilometers long.

After the earthquake struck on 26 December 2003, aerial photographs were taken to assess the damage. Archaeologists working with Iran's Cultural Heritage & Tourism Organization (CHTO) asked to see the pictures showing Arg-e Bam, the unique mud citadel that symbolized the city. "I immediately realized that the citadel should be seen as part of a larger site," said Chahryar Adle, a senior archaeologist at the CHTO and the French Center National de la Recherche Scientifique.

"I saw that lots of the qanats came to an end right up against the fault line and had the impression that there were large pools alongside them. "[Then] I discovered that the fault line itself - the tool and symbol of Bam's destruction - had once been transformed by human genius into an instrument of life."

Qanats tap water supplies deep below the feet of nearby mountains. They slope down at a slightly shallower angle than the slope of the hill, and surface kilometers away, often in areas that would otherwise be completely dry. In Bam, the qanats surfaced much closer to the water source because the fault line caused a sharp fall in the ground level. So agriculture developed along the line, eventually leading to the development of the city.

Dr Adle says the ancient irrigation system was a wonder of Iranian engineering, imitated from Chinese Turkistan to Egypt and the Arabian peninsula.

As well as helping archaeologists discover the new sites, the earthquake has revealed the historical landscape beneath the fallen citadel, revealing a layered chronology.

Eskander Mokhtari, head of the citadel restoration effort, said: "The quake opened up the ground in a way that tens of archaeologists working for decades would have been unable to do."

Dr Adle added: "The earthquake was terrible in almost every way, but it was a blessing for archaeology."