On my third day in the ancient city of Girsu, the thunder god Ningirsu spoke. A tremendous clap sounded, lightning rent the sky and the rains fell.

Across the great plain where the city once stood, at a place now called Tello, the heavy alluvial soil turned to clinging mud. At the site of  Ningirsu’s temple, built 4,000 years ago by a Sumerian king named Gudea, pools of water formed in the excavation trenches and gullies, forcing the archaeological team from the British Museum to abandon work for the day and retreat to their base, a two-mile drive across a pitted, unmade track, in the tiny village of Nasr.

This is the cradle of civilisation – the birthplace of ‘urban society’, as Sebastien Rey,…