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, the lead archaeologist of the British Museum team, puts it – although ‘urban’ is not the first word to come to mind, surveying the flat, featureless landscape, broken here and there by the spoil heaps from previous archaeological expeditions at the site.

Its period marks the transition from large village settlements to the birth of cities and states; and the development of cuneiform writing, to record information, legends and – crucially in the case of Gudea, as we shall see – dreams. It marks the birth of civil order, and the bureaucracy necessary to maintain it.